Based on your replies to comments, here's the diagnosis:
Most likely, you tripped the overdrive. Most modern cars have this, and most people don't know about it.
Look at 2 things:
- look at your shifter, there should be a second button under the shifter button. That's the button that activates and deactivates the overdrive.
- Look at your dash, there should be a light on somewhere, probably amber in colour, indicating the overdrive is activated.
What the overdrive is, really, is it either allows or prevents the car from shifting into high gear. Shifting into high gear saves gas, preventing a shift into high gear keeps you on a better gear ratio and in higher RPMs, therefore in a better powerband.
Some model years of the G6 only had a 4-speed automatic, therefore disabling the overdrive will leave you with 3 gears:
- that's why it's only shifting twice (1st, 2nd then 3rd)
- that's why the RPMs are over 5,000, you are doing 70 mph in 3rd gear!
- and that's why the cruise control isn't working, it's not supposed to be functioning with the overdrive off
Click that button once and see if the car goes back to normal. :)