Need for DRC for Gerbers?
One thing should be clear: you can't just do a DRC on the Gerbers alone. Your PCB design may have several design rules which the Gerbers don't have information about, like clearance between nets (like mains voltage and SELV) or trace width for high current nets.  

I have interactive DRC, so I get warned when I'm violating a design rule during layout. Before I create my Gerbers I run DRC again over the complete design. That's it.  

The PCB shop may/will run a DRC on the Gerbers it receives, mainly to check minimum trace widths and clearance between traces/pads. If you talked these through with your shop you should have the same design rules in your EDA software, and you shouldn't get any surprises.