Connect a Gravity Form File Upload Field to an ACF Image Field. See https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/

gravity-form-to-acf-image-field-fornt-end-posting.php

<?php
/**
 * Connect a Gravity Form File Upload Field to an ACF Image Field.
 * @see https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/
 */
$gravity_form_id = 1; // gravity form id, or replace {$gravity_form_id} below with this number
add_filter( "gform_after_submission_{$gravity_form_id}", 'jdn_set_acf_gallery_field', 10, 2 );
function jdn_set_acf_gallery_field( $entry, $form ) {

	$gf_images_field_id = 7; // the upload field id
	$acf_field_id = 'field_58ce208d568b6'; // the acf gallery field id

	// get post, if there isn't one, bail
	if( isset( $entry['post_id'] ) ) {
		$post = get_post( $entry['post_id'] );
		if( is_null( $post ) )
			return;
		} else {
			return;
		}

		// Clean up images upload and add to the ACF field
		if( isset( $entry[ $gf_images_field_id ] ) ) {
			$image_url = stripslashes( $entry[ $gf_images_field_id ] );
			$create_image = new Create_Media_File( $image_url, $post->ID );
			$image_id = $create_image->attachment_id;
			
			// Update the ACF field
			if( ! empty( $image_id ) ) {
				update_field( $acf_field_id, $image_id, $post->ID );

			// Updating post
			wp_update_post( $post );
		}
	}
}