Disclaimer: I'm not a lawyer
It means you can sell the GPLed program provided you also offer the free (no-charge) version (with no missing functionality).
So how will anyone buy your program? By offering stuff that's not GPLed in the paid version, such as a warranty.
If it's a game, you can have the game engine in GPL(and thus available at no charge), but exclude the game assets(images, audio) from the free version, assuming those assets aren't GPLed.