Having no motivation and being tired is the norm for teenagers. It's when it affects your daily life and you just can't pull yourself out of it that it becomes problematic. When you were diagnosed with GAD did the doctor offer therapy or meds or offer any advice? You say that you can't study because you don't understand it- do you mean you don't understand why you feel like this or you don't understand the work? No wonder you don't want to study if you don't understand it- if this is the case then you need to seek help from teachers, there is no point studying if you don't get the work. If you find the work fine and you just don't want to study then that is normal, no one really does want to study! Try and look at what you will get out of studying and passing your exams- you do well and the world is your oyster, you can go on to do what you want to do. You could set yourself goals and treats for when you study, like "if I study for a couple of hours today then i can go do something I enjoy". I remember when I was at school I would make a study timetable that included some study time and fun time as well. If you are so tired that you can't study/go to the gym then you should go to the doctor to see if there is an underlying health problem- underactive thyroid, vitamin deficiency etc.
If you find that no matter what you do you just can't kick yourself up the a** and get on with things then please speak to a doctor or maybe a school counsellor or teacher. There is nothing worse than struggling alone.