What happens on Itch is not typically the game industry.
Roles like "game designer" are implicit or outside the scope of a small game. You do not need that kind of analysis and prediction for small indie games. Or rather it is unaffordable.
I mean, just look at publishing on Itch. Actual professional games typically have a publisher. Itch does not even have the capacity for that abstraction layer. It is very rare to see a publisher here. It is all self publishing by the developers.
A lot of dedicated roles are non existant for games on Itch. Even for teams. There is just more roles than persons.
If you do not have a team, seeking one will probably make you look like the "ideas guy".
So for indie games, you likely will have to contribute more than a gdd. A gdd is basically a plan what to make and why.