Gilbert Gottfried actually is the voice of the duck. That's why it sounds like him.
I'm surprised GEICO cares about the shows they advertise on. They advertise literally everywhere else and spend more than half a billion dollars each year on ads. I have a subscription to mlb.tv, which lets me stream live baseball games over the internet, and during the commercial breaks mlb.tv overlays them with their own ads. So far, there have been only two advertisers to buy ad space on mlb.tv: GEICO and the various business ventures of MLB (MLB shop, etc.). I think even GEICO realizes how much their advertising has permeated the whole world, if you've seen the ad with the cavemen in the bowling alley where GEICO bought ad space on the pin-setting machine.