My 20 amp breaker in my basement to my unattached garage works fine, but when I put in a gfci breaker, it pops as soon as I flip it on, two different breakers. The only thing that is plugged into my garage outlet is my garage opener, and all the wiring is new underground in sealed conduit and 20 amp, frustrating. about ready to put all gfci outlets in the garage.
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.
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.
Have you tried to unplug the garage door opener? Since that is the only thing plugged in try that first, then if it still trips you will need to check every switch & junction box. This can even happen to pros, not long ago my apprentice wired up some lights , they did work at first but he skinned the hot in a metal box and the next day shorted. So wiring faults happen even to pros, you will need to check each branch of the circuit. And possibly the feed even in conduit. If the circuit comes in and branches going to outlets and some to lights pull the splice and see if the half that is still connected trips if no trip you would know the feed is good and the problem is someplace in the side that was disconnected you get the idea try cutting the problem in half first with the 1 device that is plugged in then to the branches or if all daisy chained disconnect it 1/2 way and see if it still trips you should have it narrowed down in 5 or 6 try’s since is is only a 20 amp circuit.