I have no idea why Gibbs would say such a thing, but your guesses don't seem that far off.
I'm not defending Gibbs. I'm standing up for the ideal that when people summarize the words of others they should try to be accurate. Gibbs never said the lack of tax returns was "evidence" that Romney had engaged in tax evasion. Your lack of respect for the truth is either a kind of immorality or carelessness.
The truly funny thing there are legitimate reasons to be concerned with what Gibbs said. But when you doctor what he said, you shine the spotlight on yourself as well.