Code for comment at https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/#comment-13186

gf-upload-file-to-post-attachment.php

<?php
/**
  * Associate an uploaded file with a post on form submission in Gravity Forms
  *
  * This is specific to a comment on my post about connecting GF with ACF
  *
  * @see https://joshuadnelson.com/connect-gravity-forms-file-upload-to-acf-gallery-field/#comment-13186
  * @author Joshua David Nelson, josh@joshuadnelson.com
  */
$gravity_form_id = 1; // gravity form id, or replace {$gravity_form_id} below with this number
add_filter( "gform_after_submission_{$gravity_form_id}", 'jdn_add_file_to_post', 10, 2 );
function jdn_add_file_to_post( $entry, $form ) {
	
	// Assumes you're creating a post on form submissions, needs the post id from the form submission
	// If you're not doing this, remove this check and replace the $post->ID below with the post's id
	// (or remove it to not associate the media file with a post)_
	if( isset( $entry['post_id'] ) ) {
		$post = get_post( $entry['post_id'] );
		if( is_null( $post ) )
			return;
	} else {
		return;
	}
	
	$gf_upload_field_id = 7; // the upload field id
	
	// Make sure we have a value for the field id
	if( isset( $entry[ $gf_upload_field_id ] ) ) {
		$file_url = esc_url( $entry[ $gf_upload_field_id ] );
		
		if( $file_url ) {
			$media_file = new JDN_Create_Media_File( $file_url, $post->ID ); // needs a post id to attach it to a post
			update_post_meta( $post->ID, 'featured_pdf', $media_file->attachment_id ); // add the pdf to the post via a custom meta
		}
	}
}

// Access the pdf elsewhere
$pdf_id = get_post_meta( $post_id, 'featured_pdf', true);
$pdf_url = wp_get_attachment_url( $pdf_id );