Some people will say it's ok to use the one you already have on your car. Some people will say it's a good idea to get a new one. Personally, I bought a new one when I put my aftermarket downpipe on my 07 WRX with 11,000 miles (give or take) since the gasket looks like it's layered and it crushes slightly when torqued. I'm a believer that it's always a good idea to use new gaskets. Besides, it's not terribly expensive. What do you have to loose, besides possibly an annoying leak?
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)