I am not a license expert- but I am pretty sure if you can only release your app as GPL (either it requires a GPL component you did not write or you are unwilling to release it non-GPL) then you can not make it an iPhone app (Apple seems to be hostile to this).  In principle there is nothing wrong with charging for GPL apps, so if it wasn't for Apple's license issues you would be fine.  However, if you wrote the app yourself (or somehow own it) then there is nothing in the GPL preventing you from releasing a non-GPL version on the iPhone app store (dual licensing).