It has to be said, so I'll say it.
Doesn't really have any relevance whether it is high or not, seeing as Roshi blew up the moon with a power level (180) only a 36 times that of the farmer with a shot gun (5), so unless that Farmer is stronger than he is letting on it means Powerlevels aren't so linear as to say something like a powerlevel of "3x" can blow 3 times as much as a powerlevel of "x" it is just too inconsistent to matter, anyway I voted A. seeing as Super Gogeta is only 2.5 billion (supposedly according to V-Jump anyway)
Power levels are silly, but putting that aside for a second, I believe the last power level stated in the series was Frieza's second form at 1,000,000. One of the official Dragon Ball encyclopaedias also listed Goku and Frieza's levels during their fight on Namek. Goku's base power was 3,000,000 and his power level as a Super Saiyan was multiplied by 50, making it 150,000,000. Frieza at 100% was 120,000,000.
So, assuming those levels are accurate, I'd have to say Gohan would have been at least twice that at Super Saiyan 2, so I'll go with 300,000,000.