Like the description states. You need to upgrade all the weapons in the game. I have included the amount of green gel needed to max each weapon out. Those are:
Handgun - 132,000 Green Gel
Magnum - 187,500 Green Gel
Shotgun - 149,500 Green Gel
Sniper - 153,000 Green Gel
Agony Crossbow - 19,000 Green Gel