So I see the thread is old now, but here’s something no one at T-Mobile will tell you(for some strange reason) — if you have a business account — everything will work as it does in the rest of the U.S. Only reason I figured this out, was because when I was there (and ran out of my 200m) — there was another girl who had T-Mobile and had no issues, after many hours spent on the phone with support — one (out of 20+ reps) suggested that perhaps she’s on a business account. 

This is absolutely ridiculous, but it is true.

As long as it doesn’t completely shut down that is good. I’m sure I will blow past 5GB in a week. 

It slows down to 3G I believe.  I maxed out my 5GB in Anchorage on that trip and was still using 3G.  I just got back from 10 days in Fairbanks and maxed out my 5GB again, and it went back to 3G.  If you're at a hotel, use your wifi and you'll get through just fine.

So it sounds like if you are on GCI you get 5GB. I’ll be up there over a week. 5GB isn’t enough. Once you hit 5GB does it allow you to use 2G or 3G speeds or does it just shut off?

I'm on a Galaxy S9+ which isn't 5G capable, but speeds are good, the same as if I were home in Hawaii.  I wasn't connecting to the Hotel WiFi because I have unlimited data.  I got the alert on roaming and was cast back into the early 2000s.  If you end up visiting here, make sure to take advantage of WiFi where available.

Thanks SteveH3032 for the info.  Well, 5GB of roaming data is certainly better than 200MB of 2G, which is what T-Mobile says is its “standard” data allowance when roaming domestically.   Are you getting 5G or 5G UC (or whatever GCI’s equivalent of 5G UC is)?

I'm visiting Anchorage and just got a text saying I have used 4GB of my 5GB roaming data.  I am connected through GCI, but it seems it is still considered roaming to the TMobile systems.  The partnership must be limited to being able to connect through and roam on GCI, not to extend TMobile service.   Their service map is clearly misleading in that you can connect, but it is not TMobile, and you're capped at 5GB.  

Any updates for 2023 on roaming data in Alaska?  Several points:

1. Based on a press release in June 2020 (see its headline at the bottom here), T-Mobile and GCI inked a new partnership that allows T-Mobile customers to roam in Alaska on GCI’s 5G network (as opposed to 2G).   If you Google “T-Mobile Alaska GCI”, the first thing in the list should be this press release, which is on T-Mobile’s website.
2. For domestic roaming, T-Mobile says it has two types of roaming partners:  standard and preferred.  Is GCI a preferred partner?  Based on the press release noted in #1 above, surely GCI must be preferred?  Does this mean that domestic roaming on GCI in Alaska allows for more than 200MB of data?  Surely so if this data is 5G?

Has anyone had any recent experience on GCI in Alaska, specifically in Anchorage?  Did you hit a data cap?  Did you get 5G?  Customer Service at T-Mobile has no clue about the roaming situation in Alaska.  All they can do is quote the standard domestic roaming policy:  200MG cap per cycle, and 2G speeds. They have no clue about standard vs. preferred domestic roaming partners.

If T-Mobile has made a strategic decision to not build out its native network in Alaska but instead rely on a roaming partnership with GCI (which does have the best coverage in Alaska), then surely T-Mobile will not apply its silly domestic data caps there of 200MG at 2G.

T‑Mobile is First with 5G in all 50 States! Customers Get 5G Coverage in Alaska Through GCI Partnership

I have a T Mobile Hotspot that is roaming in Alaska.  I think it's using GCI towers. It's crazy that I can have unlimited data when I'm in the Philippines and Thailand but not in Alaska.