A real electrician wouldn't power an attic off a gfci, that would be cruel! A diy person might, though.
If you think it might be run off a gfci then pick the most likely one and remove the outlet from the equation and wire nut the connections (just temporarily) . GFCI outlets break easily and often.
If it's not the gfci, trace the wire back as far as you can see it in the attic and take your best guess on where it's coming from, and check all the connections.
It's most likely coming from an outlet not a light or switch, but it could possibly be coming from a light ceiling box, depending on how it's wired.
You could also use the continuity function on your multimeter and a long bit of wire, even speaker wire would work.
With all power off, you'd wire nut the black lead in the attic in the switch box, what you think is the hot coming in, to the speaker wire (or equivalent).
Then go to what your best guess is on where the wire is before the attic, disconnect the black lead you think is going to the attic, and check for continuity by attaching your meter leads between the speaker wire and the black wire in question.
If you have continuity, it's the same wire.
Check where the leads plug into your multimeter, too, it's different than the voltage plug ins, also there's a "beep" setting usually that beeps continouisly if you have, you guessed it, continuity.
Keep disconnecting and checking the black wires in different locations until you find it, always with the power off. This will get you on the correct circuit, at least.
This will only tell you what you need to know if the problem is a bad connection; if the wire is broken you'll never have continuity..
