The app still won’t work for me, but maybe it’s because I’m in the U.S.?
Yep, the app is geographically restricted, and GWR does cover this on their website, "...the app will only fully function if you are within the UK. However, our website GWR.com can be used outside of the UK to purchase tickets. Any e-tickets you have purchased through the website, will be available on the app once you are in the UK."
was able to make an account and purchase tickets through GWR website, so at least I can print a paper copy. Maybe when I arrive in the UK the app will open, but I’m good if not.
So you are good to go. Once in the UK you can log into the GWR app and you will find the tickets you purchased in there. You can print and use a paper copy of your ticket as you've planned, or you can use the ticket from the e-mail on your mobile device.