How can my .pac file change dynamically based on the physical site Im located in?

Hello,

Users travel from one location to another which requires their laptops to 'load/use' a different .pac file for Internet connectivity, which is the option below.  How/where do I config Group Policy to 'dynamically' load/use the correct .pac file based on their location (or subnet I would guess)?

The specific IE Options location is the 'Use Automatic Configuration Script' below:

Thanks for your help! SdeDot