There are two requirements for the Fuel Hydrant to show, and BOTH must be valid:
- The parking must be at a gate, which by default means there must be a jetway. Outside gates, GSX by default will use a Tanker. Of course, using the GSX customization, you can override this logic, and set a parking to have Underground refueling, even if there isn't any jetway.
- The Refueling point for the airplane MUST be at least 3.40 meters high from ground. On a 737, this usually means you should place it a bit on the outside compared to the real world position, because only the outermost part of the wing is higher than that. The GSX airplane customization page is used to set this.