Don't know about Gunk, but I've been pretty happy using just Simple Green (available most places) and Purple Power (gallon jugs available at Wal-Mart, some other places have spray bottles).
Have used them for both automotive engines and my boat diesel. But I do this a couple times a year for the cars and once a season for the boat so I'm not trying to break through a long term build up of crud. No problems with the fiberglass using either one.