"Weren't you already protecting mankind?" That is kind of the point of angels, although everything they do they're supposed to do from afar. "That does sound like the reason we're here. Or at least the reason we're told we're here." He's learned by now that he can't take things at face value.

"So maybe she did send you here as a test." If that's what Gabriel wants to believe. Lucifer doesn't care one way or the other.