I would like to up grade the amps going to my garage so I can run my saws and power tools with out dimming my shop lights. I currently have a old style fuse box in the garge feed with 12ga uf cable run off a 20amp circuit breaker in the service panel. My plans are to replace the fuse box with a new G.E. subpanel 125amp 6/12. Replace the uf cable with 6ga. 4 wire run through 1" schedule 40 PVC. I figure future demands warrant overkill on subpanel & wire size. Current demands will not exceed 23amps at any given point in time since I can only use one power tool at a time. Local codes dictate GFCI outlets in the garage. My question is this: Instead of individual GFCI outlets can I install GFCI circuit breakers at the subpanel or can I use a 2 pole GFCI breaker as a feeder breaker?

Use a GFCI outlet as the first in line from the breaker to the outlet string. If both the black and the white wires are connected to the outlet AND the rest of the string is connected to the output terminals of the GFCI, the whole string has the GFCI function.

The GFCI outlet has instructions for installing it as part of a chain.

A double pole GFCI breaker at the main panel might be OK. I did have trouble with plugging a sub panel into a GFCI outlet. Because the neutral and ground were connected to the same buss at the sub panel, the GFCI would trip. GFCI outlets are relatively cheap now. You can use one for the first outlet in each circuit and feed the rest from the indicated terminals for feed through GFCI.

Are you planing 2 hot wires, a neutral, and a ground for your 6-4 cable? Check code on using the same neutral for both poles. Usually you run singles in a conduit, a hot and a separate neutral for each circuit plus one ground for non metallic conduit.