Donald had always felt something was off when he was young. Getting called "she" and "her" felt wrong sometimes and it was almost like a punch to the gut when he was called by any other name. He wasn't like his twin sister, Della; although he didn't mind the dresses and skirts when he was playing pretend, he hated them when he had to wear them any other time. Growing up in the rural area outside of Duckburg didn't give him any clue into why he felt this way, though, and it wasn't like he could ask any of his teachers or classmates. The other kids made fun of him already for living in the country.
It wasn't until he and Della came under the care of his uncle, Scrooge McDuck, that he was able to get ahold of any information on himself. Turns out, he wasn't the only person in the McDuck line that had felt this way, and the library wasn't that far from Uncle Scrooge's estate. He figured it would be better to use the public computers then to ask the librarian or Scrooge if they "have any books on feeling like the wrong gender," because he did not want to explain himself. He wasn't even sure if he could if he did run into that problem.
His soul-searching wasn't making his high school years any easier. Unable to make fun of them for where they lived anymore, kids moved to calling them the "Butch Sisters" and some other, nastier words that he didn't want to think about. Della didn't seem to mind, though; this just meant she could be more open with her crush on Daisy Duck, the most popular girl in school. The only good thing that came to him was that kids thought saying he was "basically a guy" was an insult to him, so they said it a lot.
He knew he should tell Della about what he was going through at some point, but any time they weren't at school, they were adventuring with their Uncle Scrooge, and he was always so angry at them both by the time he could talk to her that he never did. He even started convincing himself that there wasn't any point to it; it wasn't like Scrooge was going to pay for any of his medical stuff anyways, and Della already joking called him "bro". He could just live with that, never telling anyone how he felt.
That didn't stop him from introducing himself to strangers as Donald Duck, nephew of Scrooge McDuck. He saw it as a blessing that ducks in his family typically had the same body type, regardless of gender. He was constantly worried that one of these people would talk to the rest of his family about him and then not only would Scrooge and Della know, the stranger would as well. He dreaded that day and was glad that Scrooge tended to keep to himself. He didn't take his cousins into account, though, and that's how he ended up in the situation where he was now.
Donald never thought Gladstone would be the first person he would come out to. His lucky cousin was insufferable, and they were always at each other's throats. That's why he was surprised when Gladstone didn't comment on the partygoer laughing about how funny his cousin "Donald" was after he tripped for the umpteenth time. This would have been the perfect time to ruin his life like he always did. Instead, Gladstone had asked if he could talk with Donald alone for a moment and he ushered him outside.
The silence was deafening. They were both waiting for the other to start. Donald didn't know what to say. Does he play it off as a joke, or should he just tell Gladstone everything? He hadn't been planning on telling him. Finally, Gladstone breaks the silence, the most serious Donald had ever heard him. "Sooo…. Would you like me to start calling you Donald?"
This wasn't what Donald was expecting. Every scenario he had come up with resulted in his family shunning him and treating him like a freak. But here was Gladstone, the family member he trusted the least, asking him if he wanted to be called by the name he had chosen without asking why he was telling people he was a guy. Maybe Donald was wrong about him. Didn't make him any less of a jerk, though.
"Yeah, I would like that a lot."
Author's Notes: So I mentioned that Donald still liked dresses when playing pretend because he does drag a lot in the comics and I love it. Also, I would like to point out that I don't blame Della for not noticing what was happening with Donald, but someone needs to teach Scrooge that just because he became a better person from having a hard life, it doesn't mean he should give Donald a hard one. All he did was give him depression.