Writing in russian doesn't make you a russian :) I write in English, but it doesn't make me a British writer or an American writer. Shevchenko wrote some prose in russian too. It doesn't make him russian, and he never positioned himself as a russian.

As for your "I don't know anyone who thinks of Shevchenko as a russian writer:" Oh, there are many who call him russian because of his few works in this language :) In my native city, Luhansk, that's occupied by russians now, there's the monument to Shevchenko in the very center. Guess what they did? They changed the sign there, and now it's saying: "Taras Shevchenko - a great russian poet." They are just rewriting history and cultural heritage the way it's more comfortable for them, erasing everything Ukrainian so people like you didn't know and didn't hear anything of it and considered everything "great russian literature and culture" :)

As for Gogol, he is a Ukrainian who made a choice to write in russian and be russian because he saw more career opportunities there in those times.