Achievements were bandied as an option at one point but didn't make it. It would certainly be a nice addition to have some sort of Steam integration in GG.
I don't think it is possible in Lua, you would need to be using the Steam API. But there may be some options on the Steam side possibly. I have never paid to upload a game on Steam so can't say for sure. I know AGK works ok with google apps for high scores and such. You can even use your own server if you want. But it has all the commands to connect in there.
Maybe with Max being developed alongside GG, that will introduce some Steam extras that will transfer to GG as well

I definitely think it should have some support.
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