Born to Dutch and Russian-Finnish parents in the Grand Duchy of Finland of the Russian Empire, he served in the Royal Netherlands Navy during World War II, and subsequently emigrated to the United States, where he became a citizen and began his acting career on Broadway.

Gaynes' most recognized roles in cinema were that of Commandant Eric Lassard in the Police Academy series and as John Van Horn in the 1982 comedy film Tootsie.

He appeared as the curmudgeonly but lovable foster parent Henry Warnimont on the NBC series Punky Brewster; as high-powered theatrical producer Arthur Feldman on The Days and Nights of Molly Dodd; as Senator Strobe Smithers in the hit TV show Hearts Afire; and as Frank Smith, the mob boss brought down by Luke Spencer (Anthony Geary) and Laura Spencer (Genie Francis) on the soap opera General Hospital.

On April 8, 1943, Gaynes was commissioned as a seaman recruit and assigned to the auxiliary ship HNLMS Oranje Nassau, which was stationed at Holyhead in Wales.

Because of his multilingualism (he spoke Dutch, English, French, Italian, and Russian), he was detached on May 1, 1943, to the Royal Navy as a translator assigned to the convoy commodore aboard HMS Hilary, which participated during July and August 1943 in Operation Husky the Allied invasion of Sicily.

He appeared as high-powered theatrical producer Arthur Feldman on The Days and Nights of Molly Dodd, in which Gaynes' real-life wife, Allyn Ann McLerie, co-starred as his love interest, and as Senator Strobe Smithers in the hit TV show Hearts Afire.

[1][2][31] In addition to Gaynes joining the cast of his wife's series The Days and Nights of Molly Dodd in 1989, McLerie and he had previously worked together on Punky Brewster, when she guest-starred in a first-season episode.