the 2 tbps of each are for 8 ounces (1cup). (personally i find that a bit strong, but if that's what you are used to, go for it) there are 64 ounces in a gallon, or 8 cups.
2 tbps = 1/8 cup
so a cup each of the lemon and the maple syrup in a gallon will work. however, if you get a gallon jug of water, then you put two more cups of stuff in, you will overflow.
try dumping out 1.5 to 2 cups of the jug, and adding 3/4 cup of each.
or you could put 3/4 cup of each in the jug, and fill it with water, depending on ir you are buying your water in jugs or not.
technically though, you are then only drinking 6 glasses of the lemonade, not 8.
hope that helps!