I've created a patch for the gmap module. This goes under feature enhancement. It enables the use of the extinfowindow to style the info window for markers. Custom theme can be easily created. I've included a tutorial in the README.txt file in the extinfowindow directory.

This patch is compatible with the latest dev version 6.x-1.x-dev - 2010-Apr-09

To install, apply the patch. Download extinfowindow and extract under /sites/all/modules/gmap/thirdparty.

You can enable the extinfowindow at http://example.com/admin/settings/gmap .