Untested, don't use Photopost, but $GAB_ads is set at the global_start hook. Guessing here, but you'd need to either set the execution order on the
GAB
global_start plugin so that it runs before the global_start plugin of Photopost, if there is one, or look through the Photopost code and see if you need to global the $GAB_ads variable, using
global $GAB_ads; inside the Photopost function(s) because of
scope.