If you’re having trouble getting your DefaultPawn_BP to replace the DefaultPawn after setting the default gamemode to gamemode_BP and setting the new DefaultPawn_BP in the gamemode_BP you just need to restart the unreal engine editor.
I discovered that in my case there was an override in place under “Settings->World settings” which forced the default gamemode to be used instead of the blueprinted version. After setting this override to “none”, the options in the “Project settings” properly kicked in and during game start, the “DefaultPawn_BP” with the grabber component was properly spawned.