For many, the release of Google Maps for iOS was like the Second Coming. However, since Google's maps are no longer integrated directly into iOS, using some features on the iPhone -- like asking Siri for directions -- still opens Apple's Maps app.
Thankfully a few workarounds have emerged that don't require any jailbreaking or coding skills. The first, and by far the easiest is this simple tip from JailbreakNation (again, no jailbreaking required). Simply ask Siri for directions as you normally would, except add the phrase "in transit" to your query. This will open up Apple's Maps app on the Transport Apps page. Tap the "Route" button next to Google Maps and your directions will open in the Google Maps app.
Another slightly more complicated scheme from MacStories allows you to get Siri to open directions in Google Maps via Siri or Launch Center Pro by using iOS's built-in URL scheme. MacStories explains it best, so I'll let them do it. Just a word of warning though, you'll need to do a lot of manual tweaking to get this one to work, which is time consuming.