Hmmm I don't think the manga will be telling you why Gantz turned off the timer anytime soon. Because Gantz doesn't explain his actions. It's not likely he would explain to the Gantzers what happened.
Somehow I get this odd feeling that Kurono is somehow related to Gantz and Gantz is doing something to help him defeat that Oni Boss.
The invisibility was not in the first place to hide them from common people, it was to shield them as much as possible from the Vampires. So now that the Vampires have discovered them, there is no point hiding them anymore. An all out war?