Get to Schottler Medical Center and enable the "Spawn Sanchez" code. Then, look at the inside of the hospital from the bottom of the stairs and repeatedly press A on the "Cheat" option of your cellphone until you see a Sanchez appear inside the hospital. Go up to it and get on the bike. Either all the walls will disappear, all the walls will have inverse colors, or everything will turn very bright white. No matter what, Niko will be invisible along with his gun if you choose to fire it. The muzzle flash appears but the gun itself does not. You can get on and off the bike. Note: If you try to go outside the hospital you will become visible again.