Quick one since it took me all afternoon.
If you have an Area3D that won't seem to emit the body_entered signal when it hits your Gridmap, are you using Jolt Physics? Cuz thats the problem apparently.
Areas Detect Static Bodies is off by default. The issue is mentioned here:
https://github.com/godotengine/godot/issues/74300#issuecomment-1999324859
Not sure if this'll be fixed at some point, but it took awhile to come across the issue, so hopefully this helps.