Works but is hackish:

$eval = \gp\tool::function_exists('eval');
if( $eval ){
  $template_file = file_exists($page->theme_dir.'/'.$page->theme_color.'/template.php')
    ? $page->theme_dir.'/'.$page->theme_color.'/template.php'
    : $page->theme_dir.'/template.php';
  if( file_exists($template_file) ){
    $template_php = file_get_contents($template_file);
    preg_match('#\$GP_MENU_CLASSES\s(.*?)\s*\);#is', $template_php, $matches);
    if( count($matches) ){
      eval($matches[0]);
    }
    msg('$GP_MENU_CLASSES = ' . pre($GP_MENU_CLASSES) );
  }
}

Is there any better way to get $GP_MENU_CLASSES from within a plugin gadget (without the need of editing the template.php)?

Thx!

Hmm, so global $GP_MENU_CLASSES doesn't work in your situation?

global $GP_MENU_CLASSES doesn't work in your situation?

Nope, gives NULL.

Thanks! A mini-addon for testing: TestGMC.zip

Here's what's happening.

Gadget content is generated before it's actually added to a webpage. ( Generated in PrepGadgetContent() which is called by gpOutput::GetHead() )

Themes that I have made place the definition of $GP_MENU_CLASSES after the call to gpOutput::GetHead().

So, for $GP_MENU_CLASSES to be available, it should really be set in the settings.php file instead of the template.php file of a theme.

I already thought it must be something like that. Hmm.
Actually it would have been cool to know the exact menu classes on PHP side in the gadget, but on the other hand I at least know if $GP_MENU_ELEMENTS is 'BootstrapMenu" or not, which is a key puzzle piece. I'll probably get the other pieces togehter with some 'forensics'. Although regexing the values out of template.php and using eval() works it's unreliable (e.g. Suhosin etc.) and not exactly performance-wise. Funnily the most efficient way was checking the theme name against a handwritten LUT.

Thanks for checking!