Oh, I misunderstood your post. I even thought to myself, darn, at that price why did I bother replacing mine, myself.
Yea, $120 is a little steep for less than 30mins. of work.
If your lucky, the body size is the same and all the pipes, hoses & wires will be in alignment or length.
If you want to cheat, don't bother changing out the flange ring that is attached to the sink pan. Just undo the drain pipe, dishwasher drain (if applicable) and electrical. Remove the old, reverse procedure with the new.
The GD should be on it's own breaker - turn the power off before you start even though it is controlled via light switch.
If your dishwasher drains through the GD, then don't forget the remove the plug inside the fitting on the new GD.