Gmap extinfowindow
This page provides support for the Extinfowindow library for Drupal Gmap module
This is currently for Drupal 6 only.
The patch can be downloaded Here
I have bundled the library files into a tar.gz Here. This includes two sample css for Extinfowindow.
unpack the library files into your libraries folder, put the patch file into your gmap module folder and apply the patch with
patch -p1 < gmap-extinfowindow-1110048-2.patch
You will need the libraries module installed.
If you want to develop your own themed css, create a folder 'extinfowindow' in your theme folder and put your css folder in there. A good starting point would be to copy libraries/extinfowindow/themes/light to 'yourtheme'/extinfowindow/light2 and rename light.css to light2.css, then change the background color slightly (so you can see the difference), go to gmap configuration > extinfowindow and select light2, enable Extinfowindow, then test it to see if it works. It does for me.
The original javascript and support files can be found Here
The issue queue dealing with this is Here
Hope this helps, it would be nice to see this feature added to gmap.
Once support for Googlemaps API v3 comes there will be InfoBubble to consider.