There is already a deal between Valve and Facepunch. I don't know all the terms but you will need to publish your game to Steam (not exclusively).

https://sbox.game/dev/doc/systems/game-exporting/ (bottom of page)

Not sure if this was added after your comment, but it now says (in a scary red box)

> You can export your game, but you shouldn't distribute your exported game just yet.

That doesn't sound good, maybe they realized after the fact that they haven't actually worked things out with Valve?

that's pretty vague, I mean are there no real licensing terms somewhere? What am I truly signing up for when I try to use this engine? I have to publish my game on steam but in what form? Same price as I do on my own website/store? Same exact form as on other stores? Same exact time, I can't publish first on epic and only later on steam?

I doubt its much of a deal. Garrys Mod and Rust have both been wildly successful (which means Valve has made tons of money off them as well)

My point is, if I were Valve id let Garry run wild with my engine--no deal needed. Hes proven himself more than once. Just a thought!