Once the deployment process had finished, I launched RD Gateway Manager from the Server Manager. Went to the properties of the server, and selected Transport Settings. Changed the default port to whatever, I'm using 8443 in this example.
Once changed, I loaded IIS and checked the Default Web Site to ensure that the port had been correctly bound to RDWeb, which it had automatically.
Quick test ensured internal and remote access to RDWeb via the new port.
Testing any of the Published Apps, or the Remote Desktop, failed with the following.
When editing the .RDP icons, I found that there was no port.
Run Powershell as Administrator, and use the following command "Set-RDSessionCollectionConfiguration -CollectionName "CollectionName" -CustomRdpProperty "gatwayhostname:s:<YOUR HOST NAME>:<PORT NUMBER>"
Confirm this has taken effect by using Get-RDSessionCollectionConfiguration.
Once you've run that, refresh your RDWeb page (if it's still open) and download the .RDP file again, and you will find upon editing, that it will have the port under the RD Gateway Server settings.