The "oil gauge" you refer to in your post is a oil pressure gauge, not oil level indicator, and it is pretty much worthless in our cars as it doesn't actually measure oil pressure.
As for the dipstick reading, are you sure you were reading the oil level correctly? I can't imagine how the dipstick could indicate "full" if there were only two quarts in the pan.
Another possibility is that you didn't drain all of the oil from the pan. If the oil has hardened into a solid or semisolid due to sludge issues, it might not have flowed out of the drain pan when the plug was removed.