I wouldn't worry too much about the hospital assessment test score. If it's the same test we took (100 questions, multiple choice) - they make you take it when you start and again when you finish. I assure you it will not affect your paycheck or the way the patients treat you.
If you quit, you can start looking for a job in private practice. Depending on which area of the country you are in, it varies. Here in NYC, there are many jobs that only require you to have a license and a pulse. The GPR will be out a resident, which can affect the call schedule if you are in a small program. I don't know about the funding part.
But if you stick it out, I wouldn't worry about your rank from dental school. I've met dentists who were book smart yet complete idiots when it comes to treating patients. If you treat your patients well and they like you, then make the best of the GPR experience by trying to learn as much clinical dentistry as you can. Don't let the attendings bully you. All of the ones I encountered didn't care where I graduated in my class. They just wanted to see me make sound clinical decisions.
The GPR probably needs you there anyways. As long as you are around, that is more patients the clinic can treat. If you think you can get a decent clinical experience out of it, I wouldn't worry about the didactic stuff too much. You obviously learned enough if you were competent enough to graduate.
It takes a while to get used to the whole GPR thing. Unless you think you can't, give it a few more weeks before you truly decide it isn't for you.
Good luck.