As has been noted, tickets can be purchased through any train operating company.
There may be a couple issues impacting the OP. GWR recently updated their website and app, that required account holders to reset passwords. OP did you have an accout prior to the update, or have you only recently created a new account? Plus for whatever reason the GWR app does not work outside the UK. If I open the app at home, I've always gotten -
Something not working? Our app works best in the UK. If you are experiencing problems using our app outside the UK, you can use our website www.gwr.com instead. if you are attempting to sign-in to download your tickets, please try again when you are inside the UK.
Personally, no matter the TOC I've always purchased tickets through their websites, saved the delivered eticket to Samsung Notes as a backup and use the app to retrieve tickets.