One way to display a loading message is to place an image containing your message underneath the map tiles. The message is always there, but it’s not visible when the tiles have been loaded on top of it.
You have to place the image after the GMap2() object is created, because the API creates its own background for the map container when the GMap2() is created.
I suggest using low contrast, and allowing space at the side so that the message isn’t underneath the map controls.
A loading message placed underneath the tiles like this also becomes visible when the map changes zoom level or map type when new tiles need to be fetched