Not done this yet but I'd think that replacing a gasket is always better than using the old one. Might as well if it's already off and they're only about £6!
I have done this and yes there is a gasket and YES change it otherwise you'll get an awful leak. Seen it recently on my friend's 130pd Passat B5.5.
It's costs like £5-6 from Audi (bit steep for a bit of rubber, but heigh ho). Totally worth it and a relatively easy job if you have fingers that bend the wrong way to loosen the allen key bolts from the car! I think they're allen key ones anyway. Been a few years since I did it!