1" major 14 Tpi and I would imagine that it is a 55degree thread.

Check the thread pattern on the Gibbs, I would also expect it to be 55 degrees, NOT the US and European 60degrees.
To give you the safest rebarrel on this old action check this and cut the thread tight, then lap the threads in ( 320 -400 paste tighten-undo, tighten-undo and repeat until the barrel seats). It will only take 10-15 mins, but it does ensure that the pressure is evenly spread all around the breach on fireing.

Regards