It just makes a Dark Comedy?
You can run and adventure where Barney the Purple Dinosaur is a Harold of Cthulhu and performs The Play King in Yellow, to which you must stop him before he changes the local mutant popluation in zombies.
This would not be too hard of a conversion. Yes gamma world is slapstick - but if you take away some of the absurd mutations (Footus Ginormicus), maybe add some darker ones, take some dnd creatures for some space ooze (Dnd - ooze) and mi-go (Dnd - thri-kreen), and a bit of curses and madnesses. You could pull it off.
Curses could be easy - a penalty to rolling. The madnesses would be a bit more involved. The thing about Cthulhu is that the investigators go more insane the more they learn. So you would have to generate a way to track sanity. Probably the easist way would be to just give a 'hp' like stat to the players and have them make will checks or take some sanity damage each time they meet something unspeakable.
A great resource if you like Cthulhu is the game Arkham Horror, they have a great wiki
to help you on your way, and if you have not played it yet, do so!