I m coming back because i found the solution for my problem and it might help someone else
In the gamemode generated by the starter top_down content there was an default game pawn “hardcoded” towards the place in the folder of the starter BP_TopDown_character.
I changed the place of this bp to reorganize my folder and so the default game pawn was spawn instead ( the spectator pawn ?).
And after that, there is an worldsetting option ( in windows panel), that have a game mode section that have to possibility to override the gamemode and it was enable for whatever reason so whenever i was creating another bp-gamemode it was overrode by the starter bp gamemode
The solution used was creating a new C++ gamemode class, creating a new bp with this new gamemode class.
Then in this bp , setting the bp-character i wanted.
After that, in world settings, gamemode override => none.