Boundaries can be added to a map using an array of long/lat pairs:
<%= gmap :gmapid => 'bdry', :width => 500, :height => 200,
:lat => 37.319, :long => -122.017, :zoom => 12 %><br /><br />
<%= gline :gmapid => 'bdry',
:line => [ [-122.0322, 37.3344],
[-122.0217, 37.3343],
[-121.9955, 37.3208],
[-121.9963, 37.3090],
[-122.0077, 37.3104],
[-122.0214, 37.3095],
[-122.0322, 37.3124],
[-122.0322, 37.3344] ] %>