When I went to AK I sent my (frozen, vacuum-packed) catch of fish home to the East Coast by FedEx. It took more than 24 hours to arrive, but the fish was still pretty frozen when it reached me, and there were no gel ice packs. So I think you will be fine.

The amusing part of this was that I was fishing in AK with a friend who lives in the UK. She also looked up the customs issues of bringing her 20 lbs of halibut into the UK and determined that it wasn't going to be allowed. So I ended up with her fish as well as mine. We ate off that for a year, and it was great!