If one felt the need to present as the other gender, and did it all, including surgery and lifestyle changes, NOT to include sexual Intimacy of any kind. Then much later decided they had made a mistake. Post surgical, is repentance even possible? If not then is forgiveness possible? The surgical results of any "reversal" are very limited and would not be suitable to many detractors. Results studies have shown that Gender change does not change the risk for Suicide.
For me, resuming life as my previous gender is so repulsive that it is unimaginable. Does that mean there is no forgiveness and hell is the likely outcome? Sometimes I think so.
A small minority of people are born with physical attributes of both genders. Someone who does "feel" that their body does not match the gender that they wish to be is rebelling against God. Not deliberately, in most cases, but it is nonetheless going against what God has made them to be. It is a sad irony that "trust the science" is the mantra until it comes to gender choice. Science says that you do not get to choose. Yet various governments are making illegal to try and talk even young children out of choosing reassignment treatments.
I do not believe you can put the toothpaste back in the tube. However, there is no Bible verse that making that kind of mistake is the unpardonable sin. Some Christians have testified that they wish they had chosen differently. It was not that long ago that gender confusion was treated as a mental illness. Satan hates God and His creation. He does his best to corrupt everything that God intends for the human race. God created male and female. That is it.
I can't imagine that gender surgery can be reversed, at least not routinely. I've done some research that suggests it's way too hard. However, God's forgiveness is not limited by the kind of sin committed. King David could not bring Bathsheba's murdered husband back to life. But God forgave David anyway.
All God wants from us is to agree that we are sinners with no hope of saving ourselves. That is repentance. He then requires us to turn away from our former way of life and accept Christ as saviour and Lord. God will then give us a new nature, a new heart where He is able to live. He becomes our God and Father and we become His children and His people.
Lord Jesus came to set us free from all that we were and to give us new life. If we allow Him, Lord Jesus will transform us progressively, causing us to be the kind of people that God intends for us to be. That process is unique to each individual. Be assured that God loves you no matter what and that He is well able to do what is impossible for us. My advice is not to worry about specifics that you can do nothing about. Throw yourself on God's mercy. "God be merciful to me a sinner" is something that God will never reject.