Gymnastics is a performance sport and therefore looking good is important. Creativity, expression, extension and impression are common terms and goals used for high level performances. Routines are choreographed. Level of difficulty, personality and perfection are the ultimate goals.
Self defense is about effective instantaneous survival response. A person may have preferences over which leg to kick with or whether to fight stand-up or be on-the-ground. When something happens, you do not have time to warm up and therefore simplicity is gold. Factors like your relative size, speed, position, environment affect choices available – the more adaptable and agile you are, the more likely that you can resolve a threat with minimum damage.
Gymnastics build your baseline and martial arts focus on applying them effectively.