We don't shop Gap often, but I've noticed their sizes run small, so you might luck out with a pair of pants. Look for pants that have a large hem that you can let out. You can do that, even if you don't sew. You just need a seam ripper and some iron-on hem tape (called stitch witchery).
Avery has the same problem. Skinny, yet tallish. We also use the adjustable waist pants at Target - some adjust more than others. We've got Avery pulled in at 8 on both sides and they are still loose, so we tuck in shirts, which helps, roll the legs, and she tugs them up whenever necessary.