I ran into a problem using my GC so I wanted to explain what happened in the hopes it might help someone...
My dues were $2300. I had 3 GC ($1000, $1000, $900). When I clicked to pay in full, it kept rejecting the first GC I tried to use. I tried the others but they wouldn't work either. I had to select a partial payment for the amount on the GC ($1000). After that went through, I took a picture of the confirmation with my phone just in case. I then did a second partial payment for $1000 and then a third for the remaining $300 balance using the $900 GC. I also took pictures of those confirmations too just in case.
Bottom line is you can't try to pay for more than is on a single GC. I thought that maybe it would keep asking me for more payment options when the GC wasn't enough but it didn't. It just outright rejected the card.