Sundquist appeared in various film & television roles during the 1970s and early 1980s, most notably Soldier & Me, The Mallens and The Siege of Golden Hill, with guest appearances on shows such as Space 1999 alongside acting luminaries Martin Landau and fellow guest star Patrick Troughton. He appeared as Alan Strang in Equus at the Albery theatre in the mid 1970's.
His films included the 1979 British disco film The Music Machine and The Passion Flower Hotel, aka Boarding School, playing Fibs alongside Nastassja Kinski. He played Pip in Great Expectations and appeared to have a promising early career. His last acting appearance was in the 1984 horror film Don't open 'til Christmas.
Gerry’s brother Geoffrey Sundquist later wrote the children’s book Father Christmas and the Missing Reindeer, which was dedicated to Gerry. It was later made into an animated short film, David Jason did one of the voices.