What module can create a gateway or doorway popup AD page before viewing content for Drupal 7?
The Splashify module should do exactly what you need. You can specify a list of the pages you want to redirect to an ad (you can use the wildcared (*) too) and a list of different pages to use for the ads, and they're choosen either random or in order. You'll have to create the pages for the ads though. How they get from the ad page to the content they requested may require a little PHP/javascript, I'm not sure. 

http://drupal.org/project/splashify