Don't mess with robber flies.
Yay! I was really hoping to see one of these up at Sagehen.
G. latipennis and G. vermiculata seem pretty similar. BugGuide claims the latter is probably not in CA. Judging by pics on that site, it also seems that the white panels on the inner part of the forewing form a "V" in G. latipennis and a "Y" in G. vermiculata. Not sure how consistent that is.