How can I get General Aviation flight data on smartphone and/or tablet?
Garmin has a product called Connext that allows you to stream in-flight avionics data to mobile devices. They are pushing that product as a move towards connected components and a "Bring your own device" approach to cockpit automation.

So is it possible in general to receive it? No, not really. Not unless the avionics package was designed to broadcast it out. As other answers have said, a simple GPS and AHRS integration of the sensors in your tablet/smartphone can give a pretty good approximation of the data you are asking for, but things like airspeed, navigation waypoints, and other on-board sensor data are not generally available without a product like Connext. 

The other side of it is that if you are a passenger you still probably won't be able to pair with the avionics. Connext allows you to modify the flight plan via a mobile device, and I'm not sure I would want any of my passengers doing that. On top of that, pilots are trained to interpret the data at hand, you don't want a passenger seeing a faulty artificial horizon after a vacuum failure and think that the aircraft is tumbling out of the sky. 

There is a lot of security concerns as it is over being able to hack into commercial aircraft (I know you are talking about GA), but I still don't want a passenger to have the ability to connect to my avionics.