504 504 Gateway Timeout

The server did not receive a timely response from an upstream server.

When you'll see it

When a proxy, load balancer or CDN waited too long for the server behind it. The server may still be working on the request.

The difference from 502: a 504 means no answer came in time, while a 502 means the answer was bad.
Slow database queries and calls to outside services, which are the usual cause.
The proxy's timeout. Raising it only hides the problem if the request is really too slow.
Move long jobs to a background queue, and answer right away.

HTTP/1.1 504 Gateway Timeout
Server: nginx
Content-Type: text/html

Same as 502. It usually indicates a slow backend or misconfigured timeouts. Persistent 504s hurt crawl budget.