I don't have that one, but I have one that is similar (it has a print) and it runs true to size. I have found that gap is pretty consistent with my pre-pregnancy sizes
There dresses are pretty true to size, but anything with sleeves (tops, sweaters), usually runs small, so order next size up. Same thing with their pants and jeans, order next size up and nothing fitted (they are not flattering towards the end). But I got most of my maternity clothes from Gap and have been really happy.
I usually wear XS in Gap, and I bought a dress a while ago for my shower in XS. I wore it last weekend, and it still fit, but it was a little on the small side. You might want to think about going up a size if you want to wear it till the end of your pregnancy! Just my experience though
Re: How do Gap Maternity dresses run?