I have Gap jeans from 2000 that still fit me great. I think it's because they were from the pre-stretchy designer jeans days, when most jeans were 100% cotton. Anything with spandex material in it will eventually stretch out. If you must get jeans with spandex, make sure it makes up only 1% of the fabric.
My favorite gap pair disintegrated two years ago. I put them on fresh out of the dryer and they ripped in half... My mom had bought them about a year or two before she gave them to me and I'm pretty sure they were all cotton. She gave them to me in 04 and I wore them constantly. I went ahead and ordered a backup pair in early 05 for when my favorite pair gave out and they weren't the same at all! They fit way too big and stretched too much.
Ever since my favorite pair of Gaps self-destructed, I haven't found a good pair at gap. Oh well.
I've had lots of problems with Gap jeans. For the price I think they are decent, so I still buy them sometimes to keep as pairs I don't have to worry about getting dirty.
I guess this is because they're the same company, but I got a pair of Old Navy jeans several years ago and have not gone back there again... they seemed to fit very oddly and after they were washed a few times seams started to come loose and they stretched out in odd places.
I read that warm or hot water and heat from dryer will damage the spandex. Use cold water wash and cool setting on dryer to preserve life of your stretch jeans. Or possibly hang pants to dry but I find cool setting on dryer is okay for my jeans.