Welcome to the forum!
Please obscure your name (and any other personal details).
As things stand, this letter now means you are being "taken to court" for not paying your fare. As well as the amounts mentioned, you will be fined by the court. This tends to happen when GTR haven't received a reply to their original letter, as they are usually prepared to offer an out-of-court settlement to people who engage with them.
If possible please upload a copy of the reply you sent to the original letter, and the original letter itself. If there is a chance that they didn't receive your reply, it would be worth contacting them as they may still be prepared to settle and withdraw the charge.
It would also help if you explained what happened, and what the "GTR letter" mentioned was.