I'm not sure if this is a bug or should work in a different way.
On the view settings for gmap (gmap_plugin_style_gmap.inc) you can select "Marker type to use" (after "Use single marker type"). But on the output is used the "drupal" one (line 53):
$markername = 'drupal';
To use the setting should be:
$markername = isset($this->options['markertype']) ? $this->options['markertype'] : 'drupal';