low voltage from circuit - bad breaker?
These kinds of problems are very difficult to diagnose without being on site. Rather than guessing at a single fix, let me give you a process to follow.  Think of electricity as a closed water system, with hot water coming in and cold water draining out. What we are looking for is a leak in the system. I'd start at the breaker. Check the load screw to the ground/neutral buss. 120vac ok? yes, then we move further down the line to the next possible junction point. At each subsequent point, check your hot side to neutral and to ground until you find the point where there is no longer a proper 120vac measurement. When you find this point, if you have 120 hot to ground, but not to the neutral, you have an open neutral. Turn OFF the power and check the neutral to ground with an ohm meter. It should be very close to 0 ohms.  If you see no voltage to either neutral or ground, and the neutral to ground is 0 ohms, then you have an open hot.  Check the input side of the GFIC and the load side in a similar method.  If you try to follow a very straight logic, testing the three possibilites at each point,(open ground/open neutral/open hot) hopefully you will find the culprit.   Good Luck