Crossdressing is often (not always) used for the lolz. Genderbender theme (body swap, transsexuality...) can take a more serious turn. I prefer the later.
Transgender
does not fall under the Gender bender
tag, it's a separate tag all its own. But as you say, GB has more to do with body swap, magic/science transformations and the like that aren't just dressing up, and aren't analogous to real-life trans people.