Gibbs let mcgee and abbey rendez-vous amoureux, date so he would probably let Tony and ziva date, right?

At first, he didn't know that Tim , and Abby were dating. But he knows that Tony and Ziva are,I guess toi could say "meant to be."

He SHOULD let them date, but not because of Abby and Tim. Tim wasn't a member of his team when McGabby first happened, so it doesn't count.
And, even though they are ment to be, its still fun watching them being single for a while