Recently found a 1992 Sierra I am interested in buying. Because of the gas prices so high right now I dont want to get something that gets single digit gas mileage. Anybody have any idea on what I should be looking at for fuel economy? The truck is stock and has a 350 in it.

my 91 with a 350 gets between 15-19 depending driving conditions, over 300 miles to a tank

I own a 91 w/a 350, though I have a few mod's to it I get around what the last poster gets though with my lead foot it can easily get single digits.
If your looking for millage though I really would not look to the full size trucks from GMC, they are tough durable trucks that do not lend themkselves to economy as a whole, personaly if I did no camping hauling or the like I would park mine " though not sell her" and buy a toy or somthing like that.
I love my truck but there is a certain amount of pain that goes along with this, the pain in my wallet.

I get 10-15 in town , or 15-18 on the road 1990 gmc 1/2ton shortbed stepside singel cab 5.7 v8 147k on the stock drivetrain, the best mpg i evey got was 20mpg on the road, the thing is that this truck seems to get great mpg some times and then its crap.