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.

Options in the godot project settings.

Areas Detect Static Bodies is off by default. The issue is mentioned here:

Not sure if this'll be fixed at some point, but it took awhile to come across the issue, so hopefully this helps.