If you want the entire header area to be gray, you can take a look around line 1508 in your css file for the .head-wrap
and add in your background code there. The end result would look like this – http://i.imgur.com/RLQ5r8t.png
The gap you see is the bottom margin from – .site-header .wrap
on line 1518 in your css. So just remove that margin if you want, and adjust the height of your title/header area.