Enable WP AMP Plugin Suppport for Themes

To enable support for WP AMP plugin please add the following code to your functions.php file:

add_action( 'amp_init', 'invictus_gallery_amp_add_cpt' );
function invictus_gallery_amp_add_cpt() {
    add_post_type_support( 'gallery', AMP_QUERY_VAR );
}

In addition you have to add this code to add the video iframe (set in the 'invictus_video_embeded_value' custom field ) to AMP gallery posts with this code:

add_action( 'pre_amp_render_post', 'invictus_amp_add_custom_actions' );
function invictus_amp_add_custom_actions() {
    add_filter( 'the_content', 'invictus_amp_add_video_embed' );
}
function invictus_amp_add_video_embed( $content ) {
    if ( get_post_type() == 'gallery' ) {
        $video = get_post_meta( get_the_ID(), 'invictus_video_embeded_value', true );
        $content = $video . $content;
    }
    return $content;
}