Am i allowed to make modified version of gm_flatgrass?

So, am i allowed to make modified version of gm_flatgrass map?

Yes

And upload it?

it’s never stopped anyone else.

Yes, you can upload it. Flatgrass is not exactly a complex concept.

If by flatgrass you literally mean decompiling and editing the flatgrass that is presently packaged with gmod, then I am not sure how the creator(s) would feel about that, but as long as you specify it is an edit I don’t imagine anyone would really mind.