Welcome to the forum!
To be blunt your in a bit of trouble here. It sounds as though you've been evading your fares on an industrial scale over the years and if a case like this doesn't end up in court you have to wonder how serious a case would need to be to end up being prosecuted. That said GWR are normally one of the 'easier' train companies to deal with when it comes to thsi sort of thing.
What happens next is that GWR Prosecutions Department will write to you. The letter will typically take a few weeks to arrive although it shouldn't take longer than six months. The letter will say that they have received a report, are considering prosecuting you and asking for your version of events before deciding how to proceed. It is important that you engage with and reply to this letter. You might want to include the following in your reply:
- That you are sorry for what has happened
- What you have learned from the incident
- That you are keen to settle the matter without the need for court action
- Offer to pay the outstanding fare and the train company's administrative costs in dealing with the matter
GWR are usually prepared to offer an administrative settlement (commonly known as an out of court settlement) to people who engage with the process and who haven't come to their attention before. There is no guarantee of this and GWR are within their rights to prosecute you in the magistrates court if they want to.
An aggravating factor in your case is that you have done this before. GWR will be able to search your online ticket purchase history and this might make it harder and more expensive to obtain a settlement. Also be aware that GWR might deal with the single case where you were caught and then contact you at a later date about the other instances, once they have researched your ticket purchasing history.
If you are offered a settlement expect to have to pay the outstanding fares at the full Anytime rate with no credit given for the tickets you did purchase. In addition you will have to pay GWR's admin costs in dealing with the case, this is typically around £150 but is likely to be more as your case is potentially more complex for them to investigate.
Here's a link to GWR's Revenue Protection Policy which you might want to read.
Feel free to post a copy of the letter once it arrives (with personal details redacted) along with your draft reply in this thread and forum members will be happy to proof read it for you.