Insert (Google ad) token in a template
The Google Ad Manager module uses a filter for the tokens, not the Token module. You could call its internal function $content=_google_admanager_substitute_tags($text); to do the substitution for you, where you replace $text with [google_ad:ad_slot] but I am not sure if it will know enough to put all the google js in your page. Try it out, I guess.

Otherwise, you can do something like $content=theme('google_admanager_block', $gid, $google_ad); where $gid=variable_get('google_admanager_account',''); and $google_ad is your ad slot. That I've done all over the place myself with reasonable success.