CCFLs require AC power. GPOs provide DC power (same as a computer power supply does). Therefore, an inverter is always necessary to take computer power supply (or GPO) power and convert it to AC power.
It's less cumbersome (albeit less accurate) to say that the CCFL is plugged into a GPO than to say that the CCFL is plugged into an inverter which is plugged into the GPO - but the inverter's use is implied since the inverter is ALWAYS required.