A GAF of 50 is fairly low. It likely would indicate that volunteering, at least initially, might be a bit of a strain, actually, and energy is best spent on establishing a personal routine structured around stabilizing personal hygeine, healthy eating, a clean and organized living space, health habits, etc. Once a person has those down and it isn't a struggle, than yeah volunteering would be a good way to build up stamina and confidence needed to move up to job and school.

Having a step up process is hepful, but if the GAF is 50 and you feel it is fairly accurate in that it does reflect your level of functioning, it might be more helpful to focus on the things you are having trouble with first.

To take up some times maybe you could join a meetup club or something at the library that doesn't hollld you obligated to go to meetings, or something that isn't highly demanding to do on your own for the time being.