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This update (released November 23rd on Steam) isn't available on GOG at the time of writing, but here is the changelog for it:
* Significant perfomance optimization;
* Smaller build size;
* Rebalance of Act 2 difficulty;
* Bug fixes

Here are the full patch notes:

*Gameplay Changes*
Act 2 quest now in line with new Fuel Node requirements
Fixed a bug where more Pirate Putrid Snails than intended were spawning on the "Pirate's Booty" stage on Act 2
Reduced Fuel Cost on Act 2 progression from 8 to 5
Reduced the difficulty on Shooter Challenge and Tank Challenge

*Fixes & Optimizations*
Huge performance improvement
Smaller textures implemented
Fix a bug where Sneaky arm color would be wrong
Fix a bug where the game would freeze
Many broken animations fixed
Bug fixes(Fix cutscene actors quality, Fix sniper white body)
New textures and backgrounds for Cutscenes
New textures for Dialogues

Hopefully we'll get the update soon! I honestly thought the game had been abandoned, and I'm very happy to see that that's not the case.