gimnastika

1. (sport) gymnastics
   Ćerka trenira gimnastiku od pete godine. — My daughter has been doing gymnastics since she was five.
2. exercises; workout
   Svako jutro radim kratku gimnastiku pre doručka. — Every morning I do a short workout before breakfast.

Raditi gimnastiku means "to do exercises"; trenirati gimnastiku means "to do gymnastics as a sport".