Force Gravity Forms Stripe add-on to use an older version of the Stripe API, so subscriptions work

gform-stripe-use-older-api.php

<?php

add_action( 'gform_stripe_post_include_api', function() {
	if ( method_exists( '\Stripe\Stripe', 'setApiVersion' ) ) {
		\Stripe\Stripe::setApiVersion( '2016-07-06' );
	}
});