And holy CRAP they are COMFORTABLE!
I am a total GAP girl, have been for many, many years. I only own maternity pants/shorts from the GAP. They not only come in actual sizes, but since I wear GAP jeans anyways, I know they are long enough (I am 5'10") and will hold up. I did buy a new pair of jeans for this pg, but my stuff from 2009 still looks great and fits well. Anything you can get from there on sale is well worth it.
Like the others have said, I think it depends on the cut and the fabric of the dress as to whether it will fit or not. All but one of mine are empire waist, and all very flowy, strechy fabric, so they'll fit for a while, but doubtfully the entire pregnancy since I tend to be HUGEMONGOUS by 6 months. I have two really cute ones that are either cut differently or fit just right and I'm sure I won't be able to wear them at all.
I'll buy only a few since I work from home and only need some to wear around the house and maybe a couple for when we go out to eat or whatever. So not breaking the bank on clothes if I can avoid it.
Since I started this pregnancy smaller than my previous, I went out and bought a smaller pair of maternity pants (all of my first pregnancy ones are too big). I'm loving them because they fit slimmer in the thighs, but the belly is super stretchy! Even my size 12 regular jeans are getting too tight.
As for sundresses, I suppose it depends on the cut of the dress... if they are loose empire waist, then maybe they would work... but if their slim fitting through the abdomen, I highly doubt they will work. I loved sun dresses during my first and have them still in the closet ready to go for this go round!
Glad you found something you like!
I bought a couple pairs of maternity jeans at Gap the other day and really love them. I don't know what kind you got, but the ones I have are the demi-panel, so it sits under the belly and so long as I don't chunk out in the butt or thigh area, they'll last me the entire pregnancy (not that I'll be wearing jeans in the Texas heat, LOL -- will be all sundresses once May rolls around).
The only reason I went to Gap is because they actually have true sizes, not the S/M/L that every other maternity store I'm seeing has -- though I've not seen an Old Navy with maternity clothes yet (our outlet mall doesn't carry them, didn't think to look at the big mall the other day). I want something that fits me from the get-go. We're going to the big mall again in a couple weeks so I'll have to take a look-see!