You can set a marker to open automatically when the map renders by editing the map configuration.  You can do this by doing the following:

1. Go to the GMaps control panel
2. Select the Manage Maps option
3. Select the map you want to edit
4. Click on the 'Markers' tab
5. Select a marker from the 'Auto-open Marker' select list
6. Click 'Save' or 'Apply'

The list of markers defined to the map are the only ones available in the 'Auto-open Marker' select list.