Change Stripe API mode for targeted form.

gform_stripe_api_mode.php

<?php

/**
 * Filters the API mode for a specific form to 'test'.
 *
 * @param $api_mode string Either 'test' or 'live'
 *
 * @return string Returns 'test' for targeted form. Otherwise,
 *                will return the value for the $api_mode setting.
 */
add_filter( 'gform_stripe_api_mode', function ( $api_mode ) {

	// Change $form_id to the id of the form your targeting.
	$form_id = 1;
	$form = GFAPI::get_form( $form_id );

	if ( $form ) {

		return 'test';

	}

	return $api_mode;

} );