I have a real fast broadband connection but even then: nearly 8GB for a PATCH? Another "great" galaxy fuckup. I really get annoyed with gog. Bughell of a website, late patches, a forum software which many users dislike and last but not least this bad joke of a client ...
However, not the fault of the Kyn developers. Patch 3 to 4 ist only 375MB and the whole game 4GB.

So how does the game run now? Is Kyn currently up to date? Seems like the last Steam update was Oct of 2015, five updates total.

Unfortunately I can't tell. I did play the game a looong time ago (somewhere 2015 like my post) and didn't check, but it seems Kyn is abandoned from the devs. There were some unfixed problems which likely are still complained about here and/or on steam. Gog has "Update 4" online, so maybe one update is missing, wouldn't be the first time.
It seems uploading patches on gog is quite a hassle to developers and this might be the reason why gog is often missing the latest updates on several games. I'm not developing games, but I often hear the best tools to upload updates and packing new installers could be found at itch.io from a dev perspective. Maybe gog should consider some changes in that regard, if this is true.

I played through a couple of months ago and didn't come across any problems or game breaking bugs. Well worth getting the game in my opinion, I actually enjoyed playing through it a lot.

Although having said that both the devs and GOG could do with a kick up the backside regarding getting the final update here (it's still Update 4) but to be fair, Update 5 doesn't affect the gameplay at all as it only fixes the following;

"This is a small update.
- We fixed in issue in with the Settings. In some cases they weren't always saved.
- Fixed an issue with the completion difficulty level. Brutal and Legendary achievements weren't given when it should.
In order to get these trophies just load your save and make your choice again at the portal."

Thanks for the info!