I'm not sure about the current TOS of gamemaker. In a sense I think that this is the worst time to start development on a new game: there are some rumors that insiders may actually get Unity to backtrack, and depending on how all this shakes out (and how negatively this ends up affecting Unitys bottom-line), companies like Gamemaker and Unreal may take this as an opportunity to assure their users that they won't make the same mistake, and adjust their own terms. (althought apparently Unreal allready has certain protections in place were you can use any version of their engine under the TOS it was first released under. Obviously that isn't without risks: if you want to develop games for Windows 12 or whatever you'd have to use a new version of Unreal and agree to their newer TOS anyway)

Long story short, you might want to wait 1 or 2 months to let the dust settle before you pick an engine.

I believe unity had the same kind of terms as unreal and they just... removed them.

No idea how/if that's legal, but it seems to be what people are saying happened, and no-one seems to be arguing that point.

Yeah, migrating to a proprietary engine after this is I think missing the point entirely.