Remove gravity forms capabilities for non-admin users, through Gravity Form's 'gform_cap_full_access' hook.

remove_gforms_caps.php

if( ! function_exists( 'rt_filter_gform_caps' ) ) {
	function rt_filter_gform_caps( $caps ){
		if( !current_user_can( 'manage_options' ) ){
			if( current_user_can( 'gform_full_access' ) ){
				$user = wp_get_current_user();
				$user->remove_cap('gform_full_access');
			}
			return '';
		} else {
			return $caps;
		}
	}
}

add_filter( "gform_cap_full_access", "rt_filter_gform_caps" );