Robert Rietty dubbed Orso Maria Guerrini in the SPACE 1999 episode. Rietty is best known for dubbing Robert Shaw in AVALANCHE EXPRESS, Adolfo Celi in THUNDERBALL, and Tiger Tanaka in YOU ONLY LIVE TWICE. Rietty's voice pops up numerous times in season 1 of SPACE 1999 (the only good season, season 2 was overall quite abysmal and lacked the fun Italian guest stars).
Gianni Garko I believe used his own voice for the one where he fights the space dragon - Rietty is credited with his voice, but I think it's a fluke. If he did dub Garko, it's likely only for one or two brief lines.
I believe Giancarlo Prete's voice is genuinely his own. It's great to hear as he only had a mild Italian accent as well. I think the only other movies where you can hear his actual voice are DETECTIVE SCHOOL DROPOUTS, MASSACRE IN ROME, and possibly WAR AND REMEMBRANCE (haven't seen that one yet).
It's a shame these 3 didn't do more work outside of Italy. Prete especially was a very good actor and gets a nice dual role in his 1999 episode. I feel terrible he was stuck in such lousy movies like THE NEW BARBARIANS in the early 80's and then thankless small roles later on.
Also, Carla Romanelli appeared in a SPACE 1999 episode. She was in SHANGHAI JOE, MILANO ROVENTE, and COP KILLER.