I think the fit will depend on 3 main things:
1) Fabric content. The more spandex/lycra the stretchier the shorts. In my (not recent) experience, GK's smooth velvet stretched more than their nylon/spandex fabrics. Embossed/shimmer lycra seemed to stretch even less, but I am going on PAST exp. here.
2) Is the gymnast "curvy" or not. Older gymnasts, who have gone through puberty, and/or are more muscular (mesomorph) than skinny, tend to need more fabric in the hips, bottom and thigh departments! Sorry, that's just what you get for being female! However, some Nastia-shapes (ectomorphs) can probably get away with a smaller size.
3) Leotard Fit. If you are comparing short size with leo size, obviously how you like your leos, will determine whether you think shorts fit smaller or larger than the leo. The TORSO measurement is the most important for leotard. Some tall gymnasts at my club wear AL leos, just for the torso length, but in street clothes thry would probably be AS. Tall, skinny gymnasts with small-med hips may need to go down a size in shorts. However, a compact, pear-shaped gymnast may need to go up a size in shorts!
If you do not have a pro-shop at your gym, or a dance store nearby that sells GK, I would ask any other girls around your size if they have any shorts you can try on for size. Please be aware though, that fabric properties, and how much the shorts have been washed, dried and worn will all affect the fit, but at least you will get a better idea. BTW heat (eg. from drying) can damage elastic fibers, making them stretch more at fist, but gradually lose their ability to 'bounce back'.
Finally (!), the GK leos our gym sells seem to be sized somewhat inconsistantly. I have seen some hang on the rack, several
inches lower than others of the same size. Now, I haven't tried any on to compare, but just so you know
