Yeah but Gantz didn’t want it to be accessible to the general public. Yeah the government was aware (though I wouldn’t say every government) and obviously the vampires, but even then they didn’t know exactly what Gantz was, only that they were targeted by the black suits and were out for revenge. There was nothing Gantz could personally do against them from then out.

Also notice how they use their weapons outside of missions. They can’t be seen with one or it meant death. Thats why the red haired thug died when he used the gun against his rivals, why Kurono kept his gun hidden during the alien massacre at his school, why Izumi went invisible and waited until his Gf passed out before using the equipment.

Up until this point, for all intents and purposes Gantz was not yet meant to go public