dlm-gf-lock-custom-redirect-based-on-select-option.php

// Gravity Forms Dynamic Redirect
add_filter( 'gform_confirmation', 'dlm_gf_dynamic_redirect', 10, 4 );
function dlm_gf_dynamic_redirect( $confirmation, $form, $entry, $ajax ) {

	$my_label = 'What Download?'; // set this to the exact label of your download select field

	// fetch download ID from select
	$selected_download = 0;
	if ( isset( $form['fields'] ) && is_array( $form['fields'] ) && count( $form ) > 0 ) {
		foreach ( $form['fields'] as $field ) {
			if ( 'select' === $field['type'] && $my_label == $field['label'] ) {
				$selected_download = absint( $entry[ $field['id'] ] );
				break;
			}
		}
	}

	// check if we found a download, if we did replace our hidden's lead field value
	if ( $selected_download > 0 ) {
		$dlm_gf_handler = new DLM_GF_Gravity_Forms_Handler();
		$lead_id = $dlm_gf_handler->get_lead_index_of_dlm_field( $form );
		$entry[ $lead_id ] = $selected_download;

		// check
		if ( null !== $lead_id ) {

			if ( isset( $entry[ $lead_id ] ) ) {
				$download_id = absint( $entry[ $lead_id ] );

				// create download
				$download = new DLM_Download( $download_id );

				// check URL
				if ( '' != $download->get_the_download_link() ) {

					// set redirect
					$confirmation = array( 'redirect' => $download->get_the_download_link() );

					// set cookie ourselves
					$dlm_gf_handler->set_entry_id_cookie( $entry, $form );

				}
			}

		}
	}

	return $confirmation;
}