For IIS Express case follow these steps:
1) Open the IIS Express configuration file. It's located under your install directory, typically at 'C:\GpsGateServer\IIS\iisexpress_applicationhost.config'.
- I recommend saving a backup of this file before you edit..
2) Search for 'GpsGateServer' (should be inside the '<sites>' tag, at approximately line 180).
3) At the end of the '<site name="GpsGateServer" ...>' block above, you find '<binding protocol="http" bindingInformation="*:80:" />'.
Change the number '80' above to the port number you like to use instead.
4) Save the file and restart the IIS Express service.
5) Done! Launch your GpsGate Server site with the new port in the url.