Having a ND-Grad filter is a nice thing to always have, especially if you are shooting landscapes or, in this case, a city skyline.
However, if you do not have one, then having Lightroom is a very, very good alternative! Just go to the Develop Module and press "M" in your keyboard. Then, as what I did in my photos below, I adjusted the exposure of the mask to -0.5 EV. The result was splendid!
So, if you don't have a ND-Grad filter, or you simply forgot to bring one, then just use Lightroom once you load your photos to your computer.
Shots made with a Canon 7D and a Canon EF 24-105 f/4L IS USM lens. Want the best backpack for your DSLR? Check out my review of my favorite, the Lowepro Versapack 200AW, here and here.