I want to start riding a road Bike. Is it a big difference if I buy a generic one?
Both bicycles have similar components (the Gravity slightly nicer drivetrain, but with a cheapo looking seatpost, etc) and are manufactured in similar facilities in China or Taiwan. The Trek will be set up and fitted by your local bike shop, which will be very helpful if your local bike shop is competent (some aren't).

The Gravity will be set up &amp; fitted by you unless you can find a bike shop to pay $50-$150 to do that work for you-- many won't want to do that work on a bike they haven't sold.

My girlfriend has a Gravity that she's used as a commuter for the past two years in addition to some touring &amp; road rides, and it's done well— it's a perfectly competent cheap bike.

If you yourself were skilled at working on bikes, or have a friend who's willing to help you fit &amp; assemble your bike in exchange for some beers, you'll get a much better value out of the Gravity. But since it sounds like you're not in that situation, and it's fall when bikes go on sale, you'll probably do better buying a Trek/Giant/whatever from a good bike shop on clearance.