I'll echo a lot of what's been said here and add my own spin on it:
-Gender-Bender is listed as a genre. Stories with this tag have a gender change as a major aspect of the story, whether or not any of the characters are trans. For instance, in many of my stories, there's a GB tag but not a transgender tag because the gender-swapping characters don't identify as trans or don't have any obvious gender dysphoria or euphoria on either side of the change, though there are many other aspects of gender changes that can make the story interesting (such as differences in lived experience, treatment, and social expectations). In many of my stories, it's implied that the MC is trans or genderqueer, but I don't tend to focus on it, unless I have the transgender tag also included...
-The transgender tag is a subject tag for stories containing major characters who are trans or involving trans identity or trans issues. If your story doesn't involve a deep delve on (trans)gender issues or have characters who are explicitly trans, you should probably keep the tag out of your story. I would warn against ignoring GB stories with the trans tag, though, because some of the best stories of the genre involve trans characters.