*EDIT*
This cap is now replaced with GM part # 23159210
The price has increased to around $50 online.
So There are several threads about this gas cap and having it coded by a dealer for $50, or having a locksmith do it.
If you dont have extra cash or your key code you can follow my instructions and get it done in less than 10 minutes.
For those of you that do not know, this cap works with your existing side cut key. About $22 from dealer, $15 or less online
GM part # 22790846
AC Delco # GT344
The tumblers are listed as 1L, 1R, 2L, 2R, etc. these are left or right specific.
One side of the lock cyinder has 2 keyhole slots and one has 3. The side with the 3 keyhole slots is the LEFT side.
There are 2 tumblers per side, the 3rd one on the left side is for the brass retainer clip.
Grab one tumbler that has a L in it and insert it into the first keyhole slot.
Hold the tumbler centered in cylinder and insert your key
all the way.
Slide the sleeve (tab end first) over the cylinder.
If it stops at the tumbler it is the wrong one. (pic below)
If correct it should slide all the way up and rotate 90 degrees.
Once first position is correct leave it in and repeat for other left tumbler.
Then find both right side tumblers the same way.
Now carefully remove each tumbler and
remember its location.
(the 5,6,7,8 below is the order of the cut positions if you get the code from the dealer)
Insert 2 springs (I did right side 1st)
Insert tumblers.
Repeat for left side.
Insert your key, and slide over the cylinder sleeve.
Make sure it rotates smoothly.
If everything is good insert the last spring and the brass keeper till it clicks in place.
Check for full operation one last time. Once you install the cylinder into the cap it will not come out.
Align the 3 tabs in the cap, and the 3 grooves of the cylinder.
Press together, till it clicks. You may have to rotate the handle, while holding the white part of the cap. Make sure you get those 2 tips on the end of the cylinder to line up.
Insert key, and test.
Thats all, its pretty easy.