If the only behavior you've noticed that makes you think your RES is gravid is that she's kicking her legs on the basking area, it may be that she's not. RES will do all sorts of gyrations on the basking area as they warm up. My male RES has kicked similar to the way you've described, and I know he's not going to lay eggs.
How big is she (shell length without the curve)? What's her appetite like (they often lose their appetites when they're gravid? Is she trying to escape the tank)? Does she seem frantic in the tank?
She may be gravid and be displaying behavior that hasn't been mentioned, but I wouldn't necessarily think she's gravid if she's kicking on the basking area.
That said, since she is female, it's always good to be prepared by reading up on creating a nesting site.
www.tortoisetrust.org/articles/Nestsites.htm
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-