A gymnast was an athlete who performed gymnastics.
Emony Dax was an Olympic gymnast. (DS9: "Facets", "Take Me Out to the Holosuite", "Afterimage") She used to stand on her head to relax. (DS9: "Afterimage")
In 2368, upon feeling the exuberant movements of Keiko O'Brien and Miles O'Brien's unborn baby, William T. Riker commented that "He's (actually, she, unbeknownst to all) going to be a hell of a gymnast." (TNG: "Disaster")