When should I get a tune-up?

When getting a tune-up there are 2 time frames manufacturers suggest based on driving.
If you live and drive in dusty areas, places where there is stop and go traffic, snow or steep hills. The time frame is essentially half that if you tend to do mostly highway driveing.
For example;
Engine oil should be changed every 3000 miles in harsh conditions and 6000 in a highway driven car.
You should get a pretty major tune-up even thoguh some manufacturers say spark plugs last 100,000 miles....I just think they are still degraded enough to warrant replacement and just to see the condition of the plug tells a lot about how the car is running.
Some thinhgs people do not get done that they should;

Flush brake fluid
Anti-Freeze flush and fill
Transmission fluid change
any hoses that look deteriated and plugs and plug wires.

Here is one link from this sight that has some good guidance for you;

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Seee if you can get manufacturers specifications. However, I tend to dol some top end cleaning as shown in the link.
Also run a can or 2 of SEA FOAM or BG44k. You will probably have to go to a service station to get them. Run them right before you get your oil changed aqs thjey work so well that they make the oil dirty with deposits they have removed.
I hope this helps and it sounds like you and you G6 will have a long time to play around as long as you take care of it.

$100 in maintenance is $1000 in repair.....SO SO SO TRUE