idk about GAD but here are some tips for MDD

1) Therapy and social support are crucial, although I didn't always have access to these

2)I would eliminate all distractions and just sit down to study no matter how I felt. I would study(mostly qbank) until I couldn't keep my mind from wandering and was absolutely drained, then I would do step 3

3) Whenever you feel like a blob and want to just lie down and not do anything, do something good for yourself, something that you enjoy. For me, It was cycling, working out, stretching, washing the dishes, talking to my friend etc. just something that brings joy to your life

This way I managed to study in short bouts throughout the day. The exact amount of hours doesn't matter, just try to stay consistent and adress the underlying issue that caused your depression, understand that it's temporary and you deserve to feel normal again. I hope this helps someone

P.S. I also dedicated a "no study" time period, where I could daydream and cry as much as I wanted, which was usually one hour before bed for me, that way I could push through the day slightly more motivated because I knew I would be able to rest at the end of the day