MySQL triggers are indeed not supported on free hosting, they are only available on premium hosting.
However, I would personally recommend to avoid using MySQL triggers in the first place, and handle all business logic through PHP code instead. So instead of having the database update the GPA when a mark is added, add PHP code to the place where marks are added to update the GPA tables along with it.
Using only PHP code is more portable (the importance of that you see here), PHP code is more versatile than SQL, and it’s easier to test and debug in my opinion.