Well for one, as a captain Daishin would know the background of shinigami in high positions such as the kick corps commander so there's no real need for some kind of test. Two, Daishin is the kind off person to give chances based on his impressions of people. I'm just playing this as Daishin would go about it. And three i was too lazy to come up with some kind of test out mission or even have them spar. Besides Gio has obviously already shown that he's responsible enough being the kido corps commander and running squad 5 with his 3rd seat rank. It's a recommendation. Whoever the recommendation goes to should be the one to test him.