DRC tool for Gerber files
DRC is a rules-based software that requires maintenance to latest IEC design specs. These change annually, so don't expect too much from a free DRC checker unless you are doing old technology. 

Also beware that the requirements of pick and place, reflow/wave solder, and hand solder change all the rules.

You might be able to afford this:  http://wssi.com/gerbtool/