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: Empty #2: Black IPA
Secondary #1: Blackberry Rhubarb wine #2: RIS #3: Cab MerMarqeNac Wine
Kegged: Pumpkin Ale, Pre-Prohibition Lager
Bottles: Carmel Apple Cider, Big 50 Barleywine, Framboise Lambic, Barolo Wine, Berry Rhubarb Wine, Black Currant wine
On Deck: BGSA