What would I have to take off the Gold Cup to make it useable in the Traditional category?
At the very least -
Slide welded up to allow use of a mil-spec fixed rear sight in a dovetail.
No wrap-around front strap covering rubber stocks (but the standard wooden grips available on one GC model would be fine)
Trigger replaced with a "non-target" trigger (no holes drilled in it, at least)
And the "National Match barrel & bushing" would be fine as is. A bull barrel - look at a Midway or Brownell's catalog - you will find out how big a diameter a bull barrel has. It does not fit in a standard barrel bushing!
And, check the 40 ounce weight limit with an empty magazine in it for Traditional category, too! Although that weight limit is more easily achieved now (with recent rule changes), it has in the past snagged some folks who thought they had a gun that was good to go in Traditional, until weigh-in at a big match sank them!
I think you can see why BD recommended just not getting a Gold Cup if you want to shoot Traditional category, but getting a more stripped down gun.
A Colt Series 70 or Series 80 Government model, or a Springfield Mil-Spec, or Armscorp produced government spec models (made for several distributors) would be what you would more commonly see in Traditional category.
By the way, Wild Bunch shooting is NOT a high-precision game. It's more a reliability game. Having a gun that shoots 2" 50-yard groups, for example, is not necessary. Having one that goes thousands of rounds of perfect cycling and high reliability, now that is priceless!
There is a fair amount of difference between shooting Modern and Traditional. As much as there is between two handed Cowboy and Duelist styles in cowboy matches. It's probably wise to try a borrowed Traditional gun to see if you like shooting single-handed. It's not for everybody, and may not be your cup of tea if you have shot a lot of modern slide gun action matches.....
Good luck, GJ