Just following up on my earlier question on Alaska coverage. It was great! The only glitch I had was the need to reboot my phone when I landed in Alaska to get service, but I've found this to be good practice when switching countries. A native on the plane with me who uses GCI also said he always reboots his phone when going to/from the Lower 48.
All of my data/voice on GCI was treated just as if I were on T-Mobile in the Lower 48. No issues at all. Worked better than my coworker's Verizon phone up there.