In Gap jeans I generally size down. That said, for the GAP Really Skinny pants, I actually sized up, because I wanted to be able to wear these with flat boots with the pants on the outside of the boots, if that makes sense? Plus, I didn't want super-tight pants. Long story short, the GAP Really Skinny pants (not jeans, but the pants that come in black, grey etc.) are definitely skinny enough, and it's possible that you might want to size up to get a tiny bit more wear out of them. Meanwhile, even sizing up in their Really Skinny pants, I didn't find the waist to be too big, and I also didn't find them stretching out as much as their jeans do.
Hope this helps. If jeans, size down for sure. If you're talking about the Really Skinny pants, then it depends on what you want to wear them with. If only for long, long tunics and such, you can stick to your regular size or even size down, but if you want them to be a bit more functional with all kinds of outfits, I'd size up. They aren't going to look baggy, by any means.
