hey, will i be able to earn achievements with gog galaxy overlay turned off? it's pretty cpu heavy and i have a weak cpu :/

Yes, you will be able to unlock achievements, with the overlay turned off. I've played with the overlay off, and most (not all but most) achievements have unlocked for me just fine.

The problem is the achievements themselves are inconsistently unlocking. Some people have had zero trouble with the achievements and unlocked each one. Some people aren't able to unlock any (despite doing the in-game requirements perfectly), some people can only unlock some or most, but not all of the achievements.

However, the in-game overlay being off should not prevent you from getting achievements.