Having achieved in 6 years his dream of becoming a world champion, he retired from professional kickboxing in 1987 and made the leap from athlete to actor. Gruner perception is that as soon as one reaches the top in a discipline, the only way forward is downwards. So once he had become world champion, he decided it was time to explore something else. Following the lead of Reg Park and Arnold Schwarzenegger, he chose to move on to the movie industry and become a movie star.