i agree with the other guys 
All in all, obviously in the long run, your bottle neck would be with the card.. and/or possibly the wiring in the house..
BUT, putting a gigabit switch inbetween the CGN3 and the device, you should be fine. As the switch is making the gigabit link with the CGN3, and then the 10/100 is between the swtich and the device.
I am currently running the CGN3. I have a gigabit router pluged in (for a wireless AP) , and two gigabit switches, with no issues. I have a few 10/100 devices into those, who dont have an issue.
I have a linksys router into the CGN3 which is only 10/100.. so it likely does have the issue.. but its for the rogers home security, and really is just for communication, and doesnt need the speed.