Create a media file on upload, requires the JDN_Create_Media_File class from https://gist.github.com/joshuadavidnelson/164a0a0744f0693d5746

gf-upload-file-to-media-library.php

<?php
/**
  * Create a new media library entry with a file upload on gravity form submission.
  * @see https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/
  * @author Joshua David Nelson, [email protected]
  */
$gravity_form_id = 1; // gravity form id, or replace {$gravity_form_id} below with this number
add_filter( "gform_after_submission_{$gravity_form_id}", 'jdn_add_image_to_media_library', 10, 2 );
function jdn_add_image_to_media_library( $entry, $form ) {

	$gf_images_field_id = 7; // the upload field id

	// Clean up images upload and add to the ACF field
	if( isset( $entry[ $gf_images_field_id ] ) ) {
		$image_url = esc_url( $entry[ $gf_images_field_id ] );
		$create_image = new JDN_Create_Media_File( $image_url );
	}
}

gf-upload-multiple-files-to-media-library.php

<?php
/**
  * Create a new media library entries with a multi-file upload field on gravity form submission.
  * @see https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/
  * @author Joshua David Nelson, [email protected]
  */
$gravity_form_id = 1; // gravity form id, or replace {$gravity_form_id} below with this number
add_filter( "gform_after_submission_{$gravity_form_id}", 'jdn_add_image_to_media_library', 10, 2 );
function jdn_add_image_to_media_library( $entry, $form ) {

	$gf_images_field_id = 7; // the upload field id
	
	if( ! isset( $entry[ $gf_images_field_id ] ) )
		return;
	
	// Create Media File for gallery of images
	$gallery_images = json_decode( $entry[ $gf_images_field_id ] );
	if( is_array( $gallery_images ) ) {
		foreach( $gallery_images as $image ) {
			$image_id = new JDN_Create_Media_File( esc_url( $image ) );
		}
	}
}