It seems people cant read descriptions or dont know how to change the branch of Gmod they are playing.
So again: if your Gmod is crashing when done loading gm_flatgrass, its most likely because you are running Gmod in 32bit , its probably the default.

To change the branch of Gmod you´re playing, follow these steps:

-> close game
-> steam library
-> right-click gmod
-> properties
-> Betas
-> select either of the x86-64 - Chromium branches. (no big difference, one is just an update
behind)

Then your gmod should automatically update (like 200mb) and you´re done.
If everything goes well you should now be given the option to launch in x64 bit upon starting Gmod.
If that doesnt solve the crashing then comment on the addon page or look for other instructions, but please stop spamming the comments of the addon with the same problem, read the darn description. (pretty please)

For the last step please consider donating to Mee via his Patreon: https://www.patreon.com/meegmod

Id like to add that i am in no way affiliated to Mee (although i wish lmao) and this is just my own frustration with comments being spammed with the same compaint.
Thank you Mee for making such an amazing addon. (again)