MacLeod passed away overnight early Saturday morning at his home in Palm Desert, CA -- where he was surrounded by loved ones and caretakers ... this according to Gavin's nephew, Mark See. We're told he'd been in and out of the hospital these last few months with varying illnesses -- and it's unclear what exactly might've caused his death in the end.

That said, See tells us it was not COVID-related. MacLeod's ex-wife, Joan Devore, confirmed all of those details, except for the location of his death.

The guy is perhaps best known for playing Murray Slaughter on 'TMTS' -- in which he played the head writer of the fictional TV station that Mary worked at. He was a main character on the series ... appearing in all 168 episodes over seven years, opposite the lead actress.

MacLeod is also just as famous for portraying Captain Merrill Stubing on "The Love Boat" for a decade ... appearing in 250 episodes, plus spin-off made-for-TV movies from the franchise.

His TV career was pretty much second to none -- the guy had roles on hit shows like "Hawaii Five-O," "Hogan's Heroes," "Perry Mason," "The Man from U.N.C.L.E.," "McHale's Navy," "The Untouchables," "Peter Gunn," "Cain's Hundred," "The Andy Griffith Show," "My Favorite Martian," "The Big Valley," "It Takes a Thief," "Charlie's Angels," "Scruples," "Murder, She Wrote," "Oz," "JAG," "That '70s Show," "Touched By An Angel" ... and dozens more over his 7-decade career.

MacLeod was a familiar face on late night shows, talk shows, game shows, variety programs and TV specials in his heyday -- having made memorable appearances on "The Dick Van Dyke Show," "The Merv Griffin Show," "Dinah!," "The Jim Stafford Show," "Tattletales," "Celebrity Sweepstakes," "Celebrity Bowling," "Hollywood Squares," "The Mike Douglas Show," "Donny and Marie," "All-Star Family Feud," "Late Show," "The Rosie O'Donnell Show" ... and others.

GML was also a devout Christian ... and appeared in several Christian-themed shows and movies over the years -- this on top of other activism he had partaken in throughout his life.

MacLeod has over 100 acting credits to his name -- and is probably one of the most prolific actors from the 20th century. He's survived by his wife of several years, Patti, their four children and grandkids.

He was 90 years old.