This is only minor and not GOG's fault but does anyone find it strange the game doesn't tell you that you were awarded these weapons at the end? I mean if you didn't look up it online or a strategy guide back in the day, how would you know it's in your cleared save file? I know it tells you about the infinite Rocket Launcher so why not these as well?

It's a 90s pc port of a console game. They were either hit or miss.
Resident Evil 1 was a very functional port but it felt like a rushed product. Unlike RE2 and RE3 had more content and stuff than other versions.
Until digital release got main stream, console port to pc were either good or a total crap and be glad if you got a patch at all.
Sometimes you had multiple version that behave in the different ways. For example Mortal Kombat 3, the DOS version was a whole different version that the one labeled Mortal Kombat 3 for WIN 95.

Since they are additions likely created by the porting team, they might not have had the resources to produce high quality splash screen renders of the exclusive weapons and/or permission to change anything structurally. It's the kind of stuff that would've been advertised somewhere in the box. It might also have been mentioned in magazine articles and full-page advertisements.

Ah yeah, makes sense. However, it doesn't seem like it'd be hard for them to add a bit of text at the end at least. I'm actually curious now if the physical game box/manual mentioned them.

The bonus weapons are not mentioned in the manual at all! I am too lazy to look for the boxart, but i don't think they are mentioned there either.
Just look at the actual wepaon models in the game, they are completely untextured and simple geometric shapes. They were definitely rushed by the devs that ported the game. That's why there is also no addition text or help that tells you how to find out about them.

Edit: The rocket launcher was also never mentioned anywhere on official advertisement. Same with the Directors Cut Infinite Magnum.
Guidebooks and print magazine tips and tricks sections were the primary way to find out about such things back in the day.

You're not likely to miss that machine gun sitting in your inventory are you?

That's only if you load your cleared save file though.