Installed new GFCI in garage box with Line and Load wiring. Determined which wire pair on old receptacle was Load by removing a black and white wire pair, then restoring power at the circuit breaker. Old receptacle remained active, and all others now cold. Therefore wire pair removed should be the Load wires. Then wired GFCI accordingly and plug tester indicates correct wiring. Test/Reset works correctly and indicator light comes on when tested according to instructions. However, all other loads on this circuit breaker are now cold. What's wrong?
My standard advice on hooking up GFCIs (of any type) is leave the warning tape on the
LOAD terminals and hook up the GFCI
LINE only. Then, run the GFCI through its paces. Make sure it tests and resets definitively. If it has sockets, make sure you can plug in a (non-ground-faulting) load in and have it work.
Your testing of which was
LINE side was correct, since you confirmed one side was
LINE. I would hook up only that cable, and nothing else for now. Then see where you are.
If you do not get a working GFCI able to power things, then attaching anything to
LOAD would be counterproductive. Don't even think about it, it's not worth the brainwave.
Just make sure you are clear on which side is which. People do get them confused.
Shot in the dark: those other outlets are on a bootleg neutral which have no ground path after installing the GFCI.
Really, I don't like your testing methods. IMO, the load side of your GFCI is dead only when a solenoid voltage tester doesn't vibrate and show 120. If that's the case... as is usual, it's a defective GFCI.
If that isn't the case, then the problem is downstream; open every box that doesn't work.