I've never documented exactly how all of the gmap features work. However, There is a slightly outdated write up available in Help:Editing#gmap. One thing that should go there is the address and point parameters. The point parameter takes a latitude longitude point of the form (lat,lon). So, if you have the following code:
<gmap>point=(37.78197638783258, -122.46099472045898)</gmap>
you get a map centered at that point with a little balloon at that point. I plan (when i get the time ^_^) to extend this so that more than one point can be specified, and more than one balloon will show up.
The address parameter takes an address and converts it to a point on page save (NOT preview, sorry). The following code:
<gmap>address=875 Howard Street, San Francisco, CA 94103</gmap>
will be converted when the page is saved to code like this:
<gmap>point=(37.782225,-122.404076)|address=875 Howard Street, San Francisco, CA 94103</gmap>
Unfortunately, it is difficult to show during page preview where the will map actually be centered. I'll think about ways to do a preview in the future.