During a professional playing career that spanned twelve years, Gary chalked up some impressive victories which included wins over Agassi, Courier, Chang and Edberg. Retiring in 1997, he reached a career high ATP ranking of 49 in singles and 7 in doubles.

During his eight years as an ATP Tour coach, Gary helped his players get some impressive results. Under his tutelage, Swedish player Jonas Bjorkman won six Grand Slam doubles titles, an impressive number of tour doubles titles and three singles titles. Muller also helped world class players Stefan Koubek and Sargis Sargsian reach career high rankings. He has also worked with 2002 Australian Open winner Thomas Johansson of Sweden.

A serve specialist, Gary's practical coaching experience also includes four years with high performance boys and girls at a private academy in Vienna, Austria as well as training promising young players in South Africa.Gary was also on the ATP Board of Directors as a Player Representative from 2002 to 2004. He has recently taken a position as Tennis Director at the Ontario Racquet Club, something his Canadian wife, Kristin, was more than happy about.