For creating a harmonious and balanced gallery wall is good to keep things simple and hang each picture at a same distance vertically and horizontally speaking. One thing to keep in mind is to keep all frames and matting in the same color.
I posted this kitchen designed by Pablo Paniagua (above) more than three months ago. The entire house is worth to see but I love the art in this kitchenThis photographs hanging from a metal cord is a great idea to keep your walls undamaged.
For a more careless bohemian look, using old frames and not being a perfectionist freak (aka every picture perfectly straight) would do.
And last but not least is the Eclectic gallery wall. While using different frames and probably matting as well, it's always a good idea to keep a somewhat similar distance between them as well. Enjoy!
images from AD, House Beautiful, Elle Decor and Babble