gform_toolbar_menu

add_filter( 'gform_toolbar_menu', 'my_custom_function' );

add_filter( 'gform_toolbar_menu', 'my_custom_toolbar', 10, 2 );
function my_custom_toolbar( $menu_items, $form_id ) {
    $menu_items['my_custom_link'] = array(
        'label'       => 'My Custom Link', // the text to display on the menu for this link
        'title'       => 'My Custom Link', // the text to be displayed in the title attribute for this link
        'url'         => self_admin_url( 'admin.php?page=my_custom_page&id=' . $form_id ), // the URL this link should point to
        'menu_class'  => 'gf_form_toolbar_custom_link', // optional, class to apply to menu list item (useful for providing a custom icon)
        'link_class'  => rgget( 'page' ) == 'my_custom_page' ? 'gf_toolbar_active' : '', // class to apply to link (useful for specifying an active style when this link is the current page)
        'capabilities'=> array( 'gravityforms_edit_forms' ), // the capabilities the user should possess in order to access this page
        'priority'    => 500 // optional, use this to specify the order in which this menu item should appear; if no priority is provided, the menu item will be appended to end
    );
    return $menu_items;
}