Here is the full list of all 20 GNOG achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Complete PURP-L
Complete FRG-Y
Finish the game
Complete HOM-3
Complete EATER
Complete CANDY
Complete LAB-O
Complete LOG
Complete VORT-X
Complete SUB-01
Unlock every level
Leave no trace
Please don't bother the amphibians
Keep the kitchen clean
Watch what you eat
Daydream at the blackboard
Leave no worm behind
Test the astronaut for space sickness
Discover what lurks below
Experiment in the studio