How to invoke navigation on Google maps using Siri?

Yes you can

Simply append "via transit" to your Siri query. Ie: Directions to Miami Beach, Florida via Transit.

Siri will determine your target location, and will give you a choice of transit apps. Tap Google Maps and you'll be on your way.

It appears these words also work:

• by transit
• using transit
• via transit

Unfortunately, 3rd party Siri integration is something that hasn't been available to developers (yet). For example, it would be really nice to say, "Start a running activity with RunKeeper," and have it start a running activity.

The closest thing to this is to simply say "Open Google Maps." I know it may seem a little lacking, but that is all we get right now.

Here is the closest and best bet I happened to find on Google:

Append "via transit" to the end of the voice command and Siri brings up a bunch of transport apps. You can click route on Google Maps and you should be on your way.

http://jailbreak-untethered.com/how-to-use-siri-and-google-maps-for-directions-without-jailbreaking-video/

Pretty Slick!!!