You will just have a little less flavor. I doubt it will make a huge deal. You could still boil up some concentrated DME in 1-2 cups of water and add that if you are really worried about it. But I would just let it ride.
BTW, I make all my recipes to 5.75 gallons in a 6.5 gal bucket. Although I do add enough fermentables to compensate. I lose between 0.5-1 gallons of beer in trub loss and don't want to get shorted the finished brew.
Primary #1: Mugged a Monk Paters #2: Empty
Secondary #1: Blackberry Rhubarb wine #2: Empty #3: Cab MerMarqeNac Wine
Kegged: Chocolate Rye
Bottles: RIS, Carmel Apple Cider, Big 50 Barleywine, Framboise Lambic, Barolo Wine, Berry Rhubarb Wine, Black Currant wine
On Deck: BGSA, Belgian Dark Strong, Umlaut my Kölsch VII