Alright guys. This one has me stumped.

GFI for the garage was tripping last time it rained heavy. Then again today. I looked at the outlet by the HVAC outside. I could tell there was not a great seal on it. I figured water ingress and grabbed a new outdoor outlet cover and a new outlet just to be sure. I change out the outlet (Slightly wet to the touch) but the guts of the outlet box inside were pretty dry. All wires dry that I could tell. So I change out the outlet and the outlet cover.

I go back inside and flip the breaker back on (15A) it pops the GFI on the garage wall that's in circuit. OK. I unplug everything so there's zero load on the circuit. Same thing when I toss the breaker. GFI pops again.

GFI/GFCI had tripped several times when I was trouble shooting everything. So I figure maybe I just stressed the GFI outlet and went and grabbed a new one. Replace with brand new GFI outlet.

There is brand new GFI outlet & Brand new outlet / outlet cover outside. 3 of the 6 outlets are pretty new in the circuit. Within last x5 years.

Any guesses as to what is causing the breaker to trip when I reset the GFI? Any thoughts about how to trouble shoot this conundrum?

Found my multimeter but it's dead. I'll replace the batteries on it in the AM. When I have more emotional currency to spend on the issue. But outside playing outlet wack-a-mole and replacing the rest of the outlets. I'm not sure.

To Sum it up: ZERO load on the circuit. If GFI is reset the breaker pops. If I reset the breaker the GFI pops. So there is definitely a fault somewhere. Just no clue how to find it!

In my garage the builders put one of those in the loop that way, if another outlet (or device connected to it) had a short, the GFCI would pop.

Needless to say, I removed the GFCI and replaced it with a "real" outlet because the dang thing would pop when I was using a [known good] circular saw and or table saw.

With the garage GFI in its "tripped" state, plug a drill or drop light (something easy to carry around) into every outlet so you know everything on that circuit that is on the load side of the GFCI that's tripping.

Loose or corroded hot or neutral conductors in the circuit can cause GFCI to "nuisance" trip.

It could be the breaker or the new gfci outlet is bad.
Is the gfci the first outlet on the leg? Disconnecting the downstream outlets after the gfci and trying to reset will isolate the issue to the breaker/gfci outlet or the downstream circuit.

My first move would be to shut the breaker off and use a tester to check every outlet to see if there is more then the 6 outlets that you know of on the circuit, especially another gfci outlet or a second outside outlet.

The Monkey's point about loose wires is spot on. Pull each outlet and check the connections.

I have problems with chipmunks chewing romex under my porch. If you’re not in conduit it might be a critter problem.

Pickup one of those plugin 3-prong outlet testers at a store and see what it reads in each of the downstream outlets after the GFCI. You may need to temporarily remove the GFCI so the current stays on long enough to see if you’ve got any neutral or ground miss wiring in an outlet that could be causing the trip.

Damn It. Figured it out. Outdoor outlet on the back of the house was on that circuit. Had a old extension cord plugged into it that was used to charge a friend of ours golf cart. It's kinda been coiled up on the back corner of the patio out of sight / out of mind.

Jerked that cord out and whole circuit is back up and nothing tripping now.

Ghost in the Machine is squashed for now. Thanks Fellas!

Yep. Power going to ground.