Installer files are on 1.05 too already.

*EDIT:*
As Jann180 pointed out, the Halloween event did not work in the GOG version. I asked about it on the Steam forums and this was the response from a dev:
"The GOG version does not have support for custom events at this time."
*END OF EDIT*

Copied over from the Steam post:
---------------------------------------------------------------------------------------------

Hey everyone!
This patch has been a long time coming, but because we made you wait so long, we have made sure that it's a special one. It expands Serious Sam 4 with some of the features you have been requesting for a long time, even before SS4 came to be.

The 1.05 update features a Halloween special event. Find a special Jack-o'-Lantern on Death from Above, but beware. Many scary things await those who pick it up. This special event will be active through November 3, but if you miss it - shame on you - you'll be able to check it out next year, as it's not going away.

One of the highlights of this update is extra multiplayer skins. The ol' Pirate Pete is back, and so is his friend Boxer Barry. The two of them are joined by the dancing bros themselves, Groovy Gregory and Dancing Denzel, but that's not all. A brand new face has got a bone to pick with those pesky aliens, the one and only Meatless Mike.

As if dressing up in Multiplayer is not enough, you can now use all those characters in your Singleplayer adventures. And finally, all of them have their own hands in the first-person view. It's been 84 years, I know.

Find the full changelog below.

Much love from Croteam!
Danny

Serious Sam 4 - 1.05 Changelog:

What's new?
- Halloween! Halloween! Halloween! Halloween! Pick up a special Jack 'o Lantern in Death from Below, and behold as everything goes dark and new forces prey upon you. Too scared? Turn off seasonal content in the Game Options menu.
- You can now use 5 new multiplayer characters (Dancing Denzel, Groovy Gregory, Pirate Pete, Boxer Barry, and Meatless Mike) in your co-op adventures. AND THEY ALL HAVE THEIR OWN HANDS IN FIRST PERSON VIEW. Unexpected, right?
- It's Halloween, after all, so go dress up as your favorite Multiplayer character IN SINGLEPLAYER!
- You can now customize your games even further thanks to a new custom difficulty setting which balances the multiplier for higher health enemies.
- The Enemy Multiplayer option can now be set "per-player" for those of you who find joy in being absolutely destroyed in-game.
- We have updated the SS4 Soundtrack. Owners now get MP3, AAC, and FLAC versions.

Performance and stability:
- Further optimizations on SSAO quality and performance.
- Fixed a rare crash caused by an empty gadget slot in the weapon wheel.
- Fixed a crash occurring when the player has the weapon bar active during level change.

Gameplay fixes:
- Per request, ammo amounts are now multiplied for all game modes, not just Single Player.
- Witch brides are no longer multiplied. Ever.
- Lava elementals now have a name in damage dealt recap when they hurt the player.
- Netricsa entries should now get removed when skipping Man vs Beast in co-op, as intended.
- The "death screen" will now show the amount of damage Lord Achriman dealt to your now deceased body.
- You will no longer be able to use weapons while parachuting during the Ugh-Zan battle by using your filthy quick save / quick load glitch. Naughty, naughty.
- Enemy multiplier now works even after you complete the level. Go nuts!
- The weapon wheel now shows the number of available uses for every available gadget.
- Fixed a bug where the player would keep weapons taken away on chapter if the client joined after the chapter that took away weapons was started. You got to earn your stuff; nothing is free.
- You can no longer mount combat drones. You've had your fun. It's time we put a stop to it. We hate fun.
- Fixed a bug where the client wouldn't open Netricsa if picking up a gadget that host already had or opening Netricsa multiple times when picking the same gadget if the host didn't pick it before.
- Fixed an issue where real-time clock would not show up when enabled (via cvar).
- Fixed issue in TP where players projectiles would go in strange directions when enemies and NPCs are just in front of the player.
- Fixed an issue with the unlockable door in Vive La Resistance. The door will now unlock even if you finish all side missions and find all secrets. On the positive side, we know what to put on our vault now. That door is impenetrable.

Visual fixes:
- Clients should now see the Time Warp effect when playing co-op.
- Muzzle flashes will now be visible when Deagles are dual-wielded.
- Gadget targeting trajectory and highlighted area are no longer visible during a cutscene.
- The notorious T-Pose Knight is a goner. All hail the new and sexy not t-posing Knight.
- Fixed issues with the terrain disappearing on the Citadel level. It was a trap, but you didn't fall for it, so we had to fix it.
- Various visual improvements on prebaked lighting. No levels will be pink from now on.
- The Mummy will now die a more dramatic death when you kill it. Almost as if it went to acting school.
- Fixed a bunch of visibility and visual issues reported by my man Harry Poster. Thanks, brother! You're a superstar.
- Various improvements of animations across several cutscenes.

And here are the real patch notes from GOG.

As you can notice, they're mostly copy-paste, except any and all mention of a Halloween event are gone. Don't bother wasting your time looking for it. Thanks, Croteam.

You can now use 5 new multiplayer characters (Dancing Denzel, Groovy Gregory, Pirate Pete, Boxer Barry, and Meatless Mike) in your co-op adventures. AND THEY ALL HAVE THEIR OWN HANDS IN FIRST PERSON VIEW. Unexpected, right? It's Halloween, after all, so go dress up as your favorite Multiplayer character IN SINGLEPLAYER! You can now customize your games even further thanks to a new custom difficulty setting which balances the multiplier for higher health enemies. The Enemy Multiplayer option can now be set "per-player" for those of you who find joy in being absolutely destroyed in-game. We have updated the SS4 Soundtrack. Owners now get MP3, AAC, and FLAC versions.

Performance and stability:
• Further optimizations on SSAO quality and performance.
• Fixed a rare crash caused by an empty gadget slot in the weapon wheel.
• Fixed a crash occurring when the player has the weapon bar active during level change.

Gameplay fixes:
• Per request, ammo amounts are now multiplied for all game modes, not just Single Player.
• Witch brides are no longer multiplied. Ever.
• Lava elementals now have a name in damage dealt recap when they hurt the player.
• Netricsa entries should now get removed when skipping Man vs Beast in co-op, as intended.
• The "death screen" will now show the amount of damage Lord Achriman dealt to your now deceased body.
• You will no longer be able to use weapons while parachuting during the Ugh-Zan battle by using your filthy quick save / quick load glitch. Naughty, naughty.
• Enemy multiplier now works even after you complete the level. Go nuts!
• The weapon wheel now shows the number of available uses for every available gadget.
• Fixed a bug where the player would keep weapons taken away on chapter if the client joined after the chapter that took away weapons was started. You got to earn your stuff; nothing is free.
• You can no longer mount combat drones. You've had your fun. It's time we put a stop to it. We hate fun.
• Fixed a bug where the client wouldn't open Netricsa if picking up a gadget that host already had or opening Netricsa multiple times when picking the same gadget if the host didn't pick it before.
• Fixed an issue where real-time clock would not show up when enabled (via cvar).
• Fixed issue in TP where players projectiles would go in strange directions when enemies and NPCs are just in front of the player.
• Fixed an issue with the unlockable door in Vive La Resistance. The door will now unlock even if you finish all side missions and find all secrets. On the positive side, we know what to put on our vault now. That door is impenetrable.

Visual fixes:
• Clients should now see the Time Warp effect when playing co-op.
• Muzzle flashes will now be visible when Deagles are dual-wielded.
• Gadget targeting trajectory and highlighted area are no longer visible during a cutscene.
• The notorious T-Pose Knight is a goner. All hail the new and sexy not t-posing Knight.
• Fixed issues with the terrain disappearing on the Citadel level. It was a trap, but you didn't fall for it, so we had to fix it.
• Various visual improvements on prebaked lighting. No levels will be pink from now on.
• The Mummy will now die a more dramatic death when you kill it. Almost as if it went to acting school.
• Fixed a bunch of visibility and visual issues reported by my man Harry Poster. Thanks, brother! You're a superstar.
• Various improvements of animations across several cutscenes.

Thank you for the Information,I tought it was me...
I did not find the Settings for "Seasonal Content" for the GOG Version.
Very sad - Steam ONLY.....

I tried 1 day before the Halloween event ended and yes. It was just not there. The seasonal content option was there, but I never got the pumpkin at the start of level one.

I wrote on the Steam forums and a dev answered with this:
"The GOG version does not have support for custom events at this time."

Sucks but hopefully in the future :/