First. There is no reason to get going too fast and losing control going down hills. If this is happening to you, please take you bike to the local bike shop and have the brakes tuned up. Do it now before you crash.
Now the newest trail in Gwinnett is the Suwanee Greenway. Spend some time googling and more information than the link below will come up, including maps. This is planned to connect soon to another path over toward I 985. Right now it is a little under 5 miles.
http://georgiatrails.com/trails/suwaneegreenway.html