Is it unusual to be doing a 10-Page GDD for a game before have any concept art or anything?

I’m putting together a 10-Page GDD for a game I’ve been brainstorming ideas for for over a year now, but I don’t even have a team yet. I don’t even have any concept art or anything yet. Is this unusual? Because, I have no idea I should be doing this yet or not.

No, that’s definitely useful. It’s something that will change over time but it’s good to put your ideas down on paper and think things out.