You can do a search here on the GM Turbo Upgrade kit, or, Stage 1 kit as it is also called. It has been discussed many times in many threads on this forum. No need to rehash all the little details again.
It is/was a dealer available kit. Just have to check with your dealer to see.
(some dealers don't even know what it is) The kit is also available from other sources but has to be dealer installed because the computer must be reflashed.
Cost of kit varies nowadays. Around $500, but I'm sure it can be found cheaper if you shop around.
Installation by the dealer is usually 1.5-2 hrs. labor Xs their labor rate.
Gas mileage. Mine increased by about 2 mpg both city and highway. That is as long as you keep your foot out of the accelerator. Hard to do with that extra hp/tq on hand.