I solved this issue. Posting there in case anyone needs it: Be sure to update your form ID and field IDS.
// this function attaches the gallery images in the form to the post ACF gallery
add_action( 'gform_advancedpostcreation_post_after_creation_10', 'apc_update_gallery', 10, 4 );
function apc_update_gallery( $post_id, $feed, $entry, $form ) {
// gravity forms multi-file upload field ID
$gf_images_id = 7;
// I pass the {apc_media:7:ids} to a custom field called "apc_image_list"
// in the Advanced Post Creation feed which
// runs after the images are uploaded so we know the ids.
// The issue is that the merge tag above pass a string of
// comma demimited values in an array. Weird.
// So. I have to get the first array value and explode it.
$apc_str = get_post_meta( $post_id, "apc_image_list");
$apc_arr = explode(',', $apc_str[0]);
if(count($apc_arr)) {
// get the ACF Gallery Field Key
$field = 'field_5b96c3d8047f3';
update_field($field, $apc_arr, $post_id);
}
}