How about a bad garbage disposal? If it worked for over a year after you installed it then went out all of a sudden that is where I would start troubleshooting. Beyond that - why is your disposal on a GFCI outlet in the first place?

As is, if the GFCI keeps tripping you have a problem either with the diposal itself, or your wiring.

There is no need for a GFCI for an undercounter receptacle for a disposer. You must go with a standard receptacle.
Since you have a switch on the counter, you need to clip the connection on the hot side of the receptacle between the two screw connections so it feeds the disposer and the switch. You can't do this with a GFCI.

A GFCI that won't reset (even when the load is turned off) often indicates that you've got a grounds and neutrals touching somewhere. Rather than defeating the protection that the GFCI provides, I'd first look to see if you've got a wiring fault.

The disposer outlet itself, does not absolutely require a GFCI, HOWEVER if the instructions for the disposer say to use one, YOU SHOULD.

I have no idea what Tex is saying about clipping connections. None of that would seem to apply to what the poster has done. Having the switch control both sides of the GFCI would be fine.

To correct myself, you only clip the hot lead if you need a constant hot for a hot water dispenser or similar.

IMO, IF the GFCI is wired correctly, even then you are going to have a problem using a GFCI in an application where and electric motor in a disposer has short quick starts. It is also possible one of the GFCI connections is not in tight. Further, presuming you are using a metal box, it is possible on fo the screws is comming on contact with the side of the box.

Turn off the power, pull out the GFCI and check the contacts for tightness. Then suggest you wrap the contacts with electrical tape. However, bottom line, you would be better served with a standard receptacle.

"IMO, IF the GFCI is wired correctly, even then you are going to have a problem using a GFCI in an application where and electric motor in a disposer has short quick starts. "

I've never seen a GFCI breaker or receptacle trip because of short, quick starts. Why would it trip because of short quick starts?

A standard recceptacle won't as a general rule. The OP even said the disposer worked OK with an extension cord to regular receptacle. A GFCI may fail because of the constant power surge IMO.

GFCIs and induction motors often do not work well together.

Unless there is a requirement from the GD maker to use a GFCI it is not needed.

The neutral line may be touching ground somewhere (possibly even in the GD), or the GFCI has failed.

test for power at the gfci. if you have power, throw away the gfci its bad. be sure to turn on the switch when testing.
if thats the case replace it with a single reciticiple. thats one with just one plug. if it worked fine for 15 years the problem is going to be something you did.

sorry I left this sit. Then sorta embarassed at the solution...

In the process of pulling out the original gfci outlet, I noticed the 3rd wire (ground/neutral? don't remember what it's called--neither black nor white) was not connected, but couldn't tell if it had come loose while in the box, or if I disconnected in pulling the outlet out of the box.

I was stumped after having problems as posted above. I did some more research on the gfci outlets after posting the original message, but had no ah-ha! moments. Until about 3 am. I woke up and realized that I had been trying to reset the outlet with the switch turned off. Gfci outlets have to have power to them to reset. The switch controls the outlet, not the disposal. Turned the switch on the next morning, reset the outlet, and it's been running fine ever since.

Thanks for all the responses.

To Plaithumb: I had the same problem, after installing another GFCI and a standard outlet, still no power. I was ready to put in new wiring for the diposal until a light went off in my head, and I told myself! Dummy, turn on the Switch!! That was simple enough. Ha Ha. Not funny after spending 3 hours under the sink. Had to enlarge the hole at the receptacle to get to it with a jig saw. But all the posts since yours indicate that a GFCI is not needed since we have it on a switch. (Unless the Disposal Instructions call for it)

A GFCI is not required because the outlet is not in a location that the code requires it. It's got nothing to do with the switch.

Plaidthumb is almost certainly long gone, this thread was two years old.