The pressure gauge will read 12.4 atm.
Let's list what you know.
The volume doesn't change, because the metal tank has a constant volume. You can ignore the volume change, because there is none.
The only variables are pressure and temperature. This is a Gay-Lussac's Law Problem (you could also use the Combined Gas Laws, since you know the volumes).
The pressure in the tank is 12.5 atm.
This makes sense. We expect the pressure of a confined gas to increase as the temperature increases.