Determining the GetSimple version is not as straight forward as it could be, as the only available function in 2.03 - return_site_ver() - outputs the version in 2.03 and returns it in 3.0b (deprecated here). The easiest way to find out, if a plugin is installed on GetSimple 2.03 or 3.0+ seems to be

$isV3 = function_exists('get_site_version');

The probably most correct one is (only use within function, in order not to pollute the global name space!)

include(GSADMININCPATH.'configuration.php');
$version = GSVERSION;

To trigger a function myplugin_function on page deletes in GetSimple 2.03 and 3.0+ use the following code:

// GetSimple 3.0+
add_action('page-delete', 'myplugin_function'); 

// GetSimple 2.03
if (!function_exists('get_site_version') && basename($_SERVER['PHP_SELF']) == 'deletefile.php') {
  myplugin_function();
}