Strictly technically speaking, if we play on the words a little bit, yes, as you can write in GML code that runs on multiple platforms too without much tweaking of the code. We are talking mobile, desktop, various consoles...
But students in CS should know portability is not a bool related to the language used, there are other considerations as well.
In the case of GMS, I'd say the biggest one would be the price, but that's a whole other topic.
But...it really can't be compared to C/C++ in almost any way, and to my knowledge, has nothing to do with the PCTE-ISO-certification thing (not too familiar with how all this works, so I may be wrong on that one)
It's made to make 2D games, I really dont understand why people feel the urge for it to do anything else than doing just that...
though I'm trying to remember the technical term for that(turing complete or something).
Yep, but honestly, do you know of a language that is NOT Turing complete?!? To my knowledge, all modern languages are (feel free to educate me on this one!)