I'm surprised that the most popular answer on this poll has Joe Gallo working together with the commission. To me, this is not a realistic scenario for a few reasons:

1. Carlo already owed appreciation to Gallo and Colombo for the Anastasia murder and Bonanno plot, respectively. So, I don't see a motive strong enough for Gambino to whack Colombo or double cross Gallo.

2. If Gallo and Gambino teamed up to clip Colombo, then Gallo got screwed again- he was already screwed a couple of times by Profaci and Persico. Common sense dictates that Gallo would have been absolutely certain of his spoils if he whacked Colombo.
The Carlo Gambino led commission had the authority to put their approved choices into the don's role- shown by Joe Colombo, Gaspar DiGregorio, Funzi Tieri, Rusty Rastelli, etc... and they didn't.

3. The Colombo family put out an open contract on Gallo's head. Carlo could have saved Gallo, but chose not to.

4. Gallo didn't declare war on Gambino and the commission. This indicates he didn't have a deal with them. Certain defeat at the hands of the commission wouldn't have prevented Gallo from starting a war, he was fearless.