Based on your question and the original title, I assume you're not actually trying to use Google maps on WP but rather provide your users with the ability to search for Subway store locations within your app? If so, what you really need is the way to get the store locations, which you can do by calling Subway's store location webservice, and then fill and bind the pushpin collection using natively supported Bing maps control.
If I have misunderstood and you are asking how to use Google maps (or rather Google map tiles in the WP Maps control), I'm not sure that's entirely legal. See 10.1.1. in the terms of use.
If you don't need to access or manipulate the store locations data in any way and only want to provide an in-app experience for the app users, you can also simply embed the WebBrowser control and navigate to the store locator page.