Google is your friend or get someone from GTxcel to help you work around this. You're paying for their product.
Anyway, it doesn't look like using doubleclick should cost any more if you're using GTxcel. Here are some examples on GTxcel of doubleclick video ads:
http://en.documentation.gtxcel.com/display/DS/Ad+ServingThe ads should be hosted on doubleclick and just referenced by GTxcel which shouldn't cost you anything. If it actually needs to be hosted on doubleclick or they don't have the video ad feature developed yet then you're SOL.