The Nimitz has changed considerably since 1975 and the ship's midlife complex overhaul and refueling.
One deck level was removed from the island, the ship received a new main mast and radar tower. Different sponsons were constructed for the hull. Many smaller antenna mounts were added.
I'm not familiar with Gold Medals Extra detail set. Does it contain details for the new appearance, or just to take the model from the 70s to the 90s? While it certainly isn't impossible to take the ship from '75 and redo it to its current look, it will take some scratch building, particularly if the GM set doesn't contain the post overhaul details.
The Navy's Visual News Service web site, news.navy.mil has a photo gallery with lots of good photos to help you see what will be involved.
Good luck,
Mike