I have just started riding(MTB), and was just wondering if anyone knows of any good places to ride in or around Gwinnett county.

I'm not a mtn biker but some of the folks who post in the Atlanta/Decatur thread are. Maybe they can help.
http://www.bikeforums.net/showthread.php?t=234331


You could also check out SORBA's website.
www.sorba.org

Gwinnett Parks has the Yellow River trail, South Gwinnett off Johan Rd.
Winder has Fort Yargo State park, (Barrow Co), Hall County has a few in Oakwood, Gainesville College and Chicopee Woods, just up I-985 a bit. Oh! Suwanee Greenway has a short loop at George Pierce Park. Then Forsyth Co's Central Park, Roswell has Big creek trail, just off Holcomb Bridge Rd @ Old Alabama Rd. Just a few off the top-o-the-head

Bikely.com has several routes in that area.

Check ouyt these guys. They know everything in Gwinnett. Lot's pf maps too.
Greg

http://www.gtcbike.org/

Tribble Mill Park in Grayson has some trails. I've never ridden them, but I've seen others riding them.