Your chosen software (GRAMPS) gives you the flexibility to deal with this as suggested by ColeValleyGirl. When you create an event, the types in the list offered are suggestions only. So overwrite with your choice (in this case, Gender Reassignment) and create the event as usual.
The new type will be retained for future use in that tree. So you can enter dates and places (and sources) for each of "began hormone therapy", "adopted new name", "surgery" or whatever you believe is relevant. They will be distinct events but have in common the new category.
When you enter the "alternative name" in the person panel, you could create a name note that explicitly refers to the appropriate event.
Note also that the types of name (birth, aka etc) are also suggestions. You may wish to add your own category (by overtyping what is presented) to make obvious to future readers that this is not some form of typographical error. Perhaps you could use "New Identity".
Unfortunately the "gender" categories are one of the few lists that are fixed in GRAMPS. You will need to decide if it is acceptable to your family to list the person as "unknown".