Renaming/making synonym the tag could also be a good occasion to make gbc synonym before someone use it.
I deleted the gameboy-color tag from your question, rather than retagging. Platform tags are only for questions specifically about the platform. Do not add them to questions that are about games that happen to be on that platform.
There should be no need to make a tag synonym. The gameboy-color tag has only one question (yours), and there are no questions tagged game-boy-color. Remove the gameboy-color tag from your question and add game-boy-color. The gameboy-color tag will automatically be deleted since there will be no questions tagged with it.
I now see that this is not possible due to a feature that prevents users from accidentally creating new tags from typos. Moderator intervention may be required. I would mod flag on your question explaining the situation.