Like some have already mentioned, this is indeed possible through a loopback interface.
Here's a working link to this KB:
If GlobalProtect is in Azure (SAML), make sure to change the port here to your desired one:
Thank you for the reply but unfortunately I'm using Duo and not Azure as my SAML IdP and the workaround described in the KB doesn't work. Setting the Login URL seems to do nothing and setting the ACS/Reply URL creates an error saying that the ACS URL doesn't match the URL in the Assertion. Even creating multiple ACS entries doesn't help, it just sends you back to the URL in the Assertion generated by the Palo.
I tried to find out from Duo if the workaround described in the KB is non-compliant SAML that just happens to work or if the functionality of the Duo system was incomplete but I couldn't get a clear answer.
Fortunately I was able to get this working as the customer could request an additional public IP address from their ISP and we were able to put GlobalProtect on 443 on the new IP address.
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