David Gregory isn't the first to try and pin down Gates on whether or not Obama would change his withdrawal plans if things got much worse in Iraq.
Of course Obama would change his plan. Obama, a reasonable man, would do whatever was necessary for U.S. security. But Obama can't actually say out loud that he would change his mind. That would send mixed signals to the Iraqis. For Iraq to move forward, they need to know that they're in charge, that this is it -- we're out of there. The line is drawn.
Ultimately, Gates said that everything has been well thought out and the chance that the withdrawal plan will need change is remote. Gates also compared Obama to Bush. Fascinating. See that here.