Saying "It wasn't Giffords, it couldn't have been Giffords, nothing she said or did could possibly have had anything to do with it, he was just insane" seems to prematurely eliminate a lot of possibilities.
Her politics may have had absolutely nothing to do with this incident. This gathering could have just been the high visibility event to get the shooter the infamy he was probably seeking.
Possibly.
On the other hand, it doesn't sound as though it was particularly high-visibility. No cameras, no live news coverage--if he was looking chiefly for a splash, doesn't it make sense that he'd wait for something showier?
"But whether the Constitution really be one thing, or another, this much is certain--that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist." --Lysander Spooner, 1867
