Your devices are NEVER broken -- Let's be clear about this. GFCI is a fault-detection device. It detects ground faults, mainly in appliances, but it can happen in wiring too. However, **ground faults *in appliances* only happen to other people. YOUR appliances never ground-fault!** So definitely leave everything in the garage plugged in, and don't go unplugging anything to troubleshoot. Narrow it down --- What's left? It could be a hardwired appliance like a lamp, *but it would have to go to a neighbor's house*, because *your* lamps don't fail! It could also be a fault in the wiring. A hot-ground hard fault would trip a regular breaker. A soft fault (leakage) might not, but would surely trip a GFCI. This might happen if a box was getting wet, or if outdoor wiring was leaking (perhaps because it is not rated for outdoor use, e.g. NM "Romex" outside or buried). A neutral-ground fault will not trip a breaker, but will trip a GFCI. This could be a bootlegged ground, or a neutral screw touching a ground wire at a receptacle. Look at the cable to the garage. Is it /3 cable with a black, red and white wire? **That's the problem right there**: This type of wiring is incompatible with *single-pole* GFCI breakers. First, the black and red wires need to be on a 2-pole breaker with common shutoff, *regardless*, for safety - they must shut off together. Then, you need to use a 2-pole GFCI breaker, because it need to measure all 3 wires - both hots and the neutral.