Route 53 Health Checks are impossible to fully enable to AWS API Gateway when the api on the gateway has API keys enabled. Why? Because the request to the gateway must send the "x-api-key" header with a known value.
To my knowledge, at best you can configure an HTTP (not HTTPS) health check to the path desired. This request will be responded with a 301 redirect. The 301 is accepted as "healthy" by the default rules of a Route 53 Health Check.
If you were to configure an HTTPS health check, there would be no 301 redirect. Instead you would immediately receive a 403 Forbidden. This would fail the health check.