Just use a wire nut that support three wires. Wire nut the two main wires with a short pig tail then wire to the one side of the GFI.

the more important question is why do you have two different gauges of wires on the same circuit?

do you have 12 and 14 AWG on the same circuit? If so what size breaker is protecting this circuit? If it's not 15 then you have a bigger issue you need to address.

One is the wire from the original build, 50 years ago. The other is new. I am going to see if I have enough room for a pigtail.

I am not aware that wire gauges in use today are any different from that of 50 years ago. I was involved in the trade at that time. I have every NEC including 1959 and since. I need more information before offering advice.

"One is the wire from the original build, 50 years ago."

But you have the breaker sized for the smaller wire, right?
Which would also mean it was a waste to run the bigger wire.
Which also probably means you have a violation by not having a 20A circuit in a bathroom or kitchen. Unless of course this is a different area.

I personally don't like the sound of this whole thing.

must be that kind of wire we used to use back in the day that shrinks with age....

do the right thing, check into it please and don't just slap on the GFI and call it good. Is your house and safety really worth the risk?

The difference is only .010-.015. Thanks for the suggestions. I pigtailed it and have completed the project.

"The difference is only .010-.015."

Wire diameter?

The difference is than #14 is only rated for 15 amps, while #12 is rated for 20 amps.

But hey, at least you are insured by your homeowner's policy against your own mistakes.

I am not a rookie. I know the difference between 12 and 14 just like you do. So stop with the rash remarks. I dont need them.

Obviously you've found out how to make it work. I think the concern is that if you have 14 gauge wire in the circuit, you should verify that the breaker is only 15A. It isn't a problem you are causing with your fix, but it is a potential problem you uncovered and you should address it. Adding a GFI has made the situation slightly safer than it was, but that is no reason to ignore a possible fire hazard.

In the bigger picture, if you see something like that in a house, it should make you suspicious of all the other electrical work the prior owners did.