It'd increase you gas mileage...anything that lets the engine breathe better will increase it.
Dont just get a downpipe..you'd be much better off with a set of headers. Speed Daddy makes a decent set
1997 1998 1999 2000 PONTIAC GRAND PRIX 3.8L V6 HEADERS - eBay (item 170469647635 end time Mar-02-11 16:41:03 PST)
All you would need then is a highflow cat welded in.
The headers outflow the stock exhaust manifolds greatly, as well as the downpipe.
Plus, you'll pick up even more gas mileage, and help get rid of any knock retard you currently have.
Going from stock exhaust, to those headers, I grabbed about 5mpg in the city, and about 10 on the highway..seen over 35mpg on the interstate cruising.
But there are write-ups on how to install those headers. They are one of the single best "bang for the buck" mods you can do
