What happens when you try?
I don't think your provider's "call forwarding" is involved in a Google Voice outgoing call. You dial the number in the Google Voice app, which informs the Google Voice system that your call is coming, then your iPhone dials a Google Voice relay number that connects you to your destination number. Sort of a three-way call between Google, you and the party to whom you are speaking...with Google in charge of the three-way hookup.
I can make them with no problem on StraightTalk, which is using Tracfone/AT&T towers.