Hello Pretendsleep and welcome to the forum, well done on looking for help.
Unfortunately only your father can make the decision to stop gambling. From what you say you have already made sure he is aware of all the support that he has available.
Now you need to concentrate on you. You don’t have to do this alone, you can get a lot of support here both on the forum and in the groups. Also of course the support you have at college.
If he doesn’t decide to address his addiction then you might have to make some of those tough decisons you are talking about, there are no wrong or right answers there – if it’s the right thing for you then its the right thing! Using support will put you in a stronger position to make those decisons.
Maybe show your mum this site as well – obviously she will also have to make her own decisions.