How is possibile to fallback to another API service if the first try fails with status code of 429 or 500?
Consider this situation: The gateway should first try a microservice host, but if it timeouts or answer a non 2xx status code it should try a next one.
This is not supported in Express Gateway out of the box. If a request is failing, you'll get a failed request.
Right now you can specify multiple urls for a serviceEndpoint that will be used, but in a round robin manner; therefore you'll still get an error first, before trying again with the second service.
Express Gateway could — and maybe should — handle such case. In meantime I'd suggest you to look to other alternatives offered by your infrastructure. Consul could be the way to go.