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 wiringwiring* 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.
Another potential problem is if two separate circuits (not MWBCs) unintentionally mix their neutral wires. That's a perception problem among installers: you should intermix all grounds from multiple circuits, and some people think they should intermix neutrals too. Or they need a neutral wire, so they grab one from another circuit. That's very common with smart switches and in boxes with 3-ways. (Thanks DoxyLover).
All these problems should be observable by opening up the junction boxes (with power off and tested to confirm off, obviously).
* It's called a Multi-wire branch circuit and there's nothing wrong with it, however it does require knowledge and special handling, GFCIs being one such case.