Index: trunk/src/org/openstreetmap/josm/Main.java
===================================================================
--- trunk/src/org/openstreetmap/josm/Main.java	(revision 11987)
+++ trunk/src/org/openstreetmap/josm/Main.java	(revision 11988)
@@ -1002,10 +1002,12 @@
          */
         List<Future<?>> downloadGps(String param) {
-            JOptionPane.showMessageDialog(
-                    Main.parent,
-                    tr("Parameter \"downloadgps\" does not accept file names or file URLs"),
-                    tr("Warning"),
-                    JOptionPane.WARNING_MESSAGE
-            );
+            if (!GraphicsEnvironment.isHeadless()) {
+                JOptionPane.showMessageDialog(
+                        Main.parent,
+                        tr("Parameter \"downloadgps\" does not accept file names or file URLs"),
+                        tr("Warning"),
+                        JOptionPane.WARNING_MESSAGE
+                );
+            }
             return Collections.emptyList();
         }