Converting between Gap and Old Navy trips people up because the two brands are cut differently. Gap runs a little large, while Old Navy runs large thanks to generous vanity sizing. Old Navy generally runs larger than Gap, so the same body usually takes a smaller size label at Old Navy. The converter above corrects for this automatically by reading the body-measurement range behind each size rather than matching the letter on the tag.
As a quick guide: a size S in Gap converts to about a S in Old Navy, a M lands around a M, and a L is roughly a L. When your measurement falls near a boundary the tool flags it as a between-sizes case — worth checking the garment's own measurements before you order.