We went through this process last year, for a child record that had been entered wrong a number of years ago (inactive family, so nobody noticed, until somebody finally noticed). We were told by GSD that the Bishop is the only one who can do this, and it must be done through his account in LCR => Help => Contact Us. If the Bishop is logged in, it should take him directly to the ServiceNow (aka GSD) website. If you'd rather not use the interactive site to create the ticket, the Bishop can call the ServiceNow phone number or email listed on that website home page.
We actually had to submit a ticket twice -- the first time they said it was fixed, but it never appeared that way on the member record, so we asked them to do it again.
As you can imagine, the Church wants to make sure they understand the nature of the gender change request, I imagine this is why only the Bishop can request this type of change.