Can GC's be used with Vouchers?

I was called by UA to change flight times prior to our last family trip and was offered $200 per ticket (5) to do so. I was given a promo code and 5 pin codes.

I've never used voucher codes like these before to purchase flights, but I do want to use them later this year.

My question is: can I use UA gift certificates along with these voucher codes?

Thanks for any help.