Should've checked the radar beforehand and figure out roughly where the alien was. The rifles he brought can shoot up to 1km away, and Gantz has always teleported them close to their target, except for the Onion. He could've sniped the alien instead of jumping over.
Even then, he could at least throw the weapon over if he can't jump while holding them instead of leaving them on the floor. Looks like his survival instinct is gone now with no one to show off for.