No, you would actually have about 4 wpg.
So here's the rundown:
The watts per gallon rule is based on the wattage of straight tube fluorescent bulbs; so, a total of 36 W of straight tube fluoros over your 20 = 1.8 wpg. Incandescents put out way less light per watt used than fluoros, so the rule does not apply to them. CFL's are slightly less efficient than tube fluoros, but are close enough. In this case the number you need is the actual wattage, not the 'incandescent equivalence' wattage.
CFL's usually are printed with an incandescent equivalent wattage. So the manufacturer intends your 9 W CFL to be equivalent to a 40 W incandescent. But for WPG purposes you have 9 W.
As far as the fixture capacity, again it is only the actual wattage that matters. This is convenient because a screw-in fixture that can only take a dim 25 W incandescent can take a quite bright 25 W CFL. These are usually sold as '100 W incandescent equivalent'.
But the WPG rule is really only helpful with relatively large tanks. You will need a higher wattage to achieve the same amount of illumination in a small tank. Even so, a 25 W CFL is probably overkill in that fixture, though there's no harm in trying it. I think I would go with an 18 W CFL.
