Can I specify the ethernet port when using the webgate on the Magelis XBTGT?

Yes. There are two parts to Webgate: Port 80 (default) is to connect to the Web Server, Port 6000 (default) is for the data portion (showing the values in the variables) which is the heart of Web Gate.

The Data Sharing option is above the Webgate option in the property inspector on the target. When you enable Webgate, Data Sharing is also enabled automatically.

The specified port in the Data Sharing section is where the Webgate client would go to get the data. In the example below port 7001 is the data port for Webgate.