The first step I would do is to convert gallons into cubic feet.

So, 1 gallon = 0.133681 cubic feet. 10 gallons = 1.33681 cubic feet.

Now, there would be a few ways to solve your problem. Remember for an ideal gas, PV=nRT. Assuming you keep Temperature constant. So, pressure and volume are related linearly. Increase the pressure by 10x in a fixed container, and the volume of gas held by that container increases by 10x.

So, I would convert PSI to ATM (or BAR).

1 ATM = 14.69595 PSI.

So, 120 PSI = 8.17 ATM. Now, typically PSI does not include the static atmospheric pressure, so to make the calculations easier, add 1 more ATM, so one gets 1 ATM == 9.17 ATM.

Multiply that times your volume above, 1.33681 Cubic Feet, and you get:

9.17 ATM x 1.33681 CF in container = 12.3 Cubic Feet of air contained in the 10 gallon container at 120 PSI.

I'll leave the calculations at 7000 PSI to you.