I skipping the "coming out" portion and going straight to the "therapy" portion.
I told my grandma I wanted to go to therapy, and she was okay with that. I didn't specify why though.
I printed out a list of gender therapists in my area, with "Gender therapists" written on the top so I don't lose it, and gave it to her. She said she's gonna look it over and call some.
I doubt she or my grandma would understand what "gender therapy" refers to by itself. I'd have to explain, and come out formally.