If you do increase the SG by adding sugar, keep in mind it may make the wine even more unbalanced... thin flavor with higher alcohol. To wineforfun's point, the kind of kit would be important to consider (big red or light white). I'd just let it run as is and see how it turns out. If it's too thin to your liking you could always get another kit and blend. Or you could get another kit immediately and add some of the juice from that to this batch and make a 3 gallon from what's left over (save the rest for back sweetening something later). But that's introducing a lot of opportunity for things to deviate from the kit as well. Like I said...I'd just let it ride.