Can you load Gerber files back into a PCB layout designer such as Eagle?
As of Eagle 7.5, this is indeed possible. It's a simple case of File-&gt;Import-&gt;Gerber from the layout editor. There is a video demo on YouTube.

As others have mentioned, there is no real DRC checking possible of the imported file as the Gerbers contain no information on nets and connectivity. Eagle simply imports all of the various shapes and lines onto the copper layer.

It does however allow you to make modifications manually (with great care) and then re-export as a Gerber again once you are done.