I can't find something like that in the official EULA.
I can't either. What I can find in the latest license agreement of GMS, is:
4. PROHIBITED ACTIONS
...
ii. Do not ...create derivative works
The term "derivative work" is well described for example here:
https://en.wikipedia.org/wiki/Derivative_work.
In short, you don't create a derivative work, if you create something with, in relation to the original work, substantial new ideas and concepts developed by yourself.
Whether this will be competitive to the original work or not does not matter at all (think of an artist painter, wo derived a substantial new painting from another painters work. Of course the new paintig is in competition to the first work in the arts market).
As "derivative or not" is an undefined legal concept, a judicious layman can build a well founded opinion about the relationship of two works himself, but the final decision in case of a lawsuit then behooves to a judge. At least from german jurisdiction I know that one can get surprises here in both directions.