Nitron was their word for "Pearl" so Nitrons were wrapped drums. Some of the finishes, like Cadillac Green were usually painted onto clear wrap so it can be confusing.
I have a Progressive Jazz snare in Black Lacquer. I think you could get custom lacquer colors done (not 100% sure about Gretsch but Ludwig would definitely do it) whereas the nitron finishes were limited to whatever was cataloged.
I think the USA Custom line started around 1983 when Charlie Roy took over. Their attitude was that all their drums were "custom" built...which was more or less true since they were selling them in such small numbers by that point. I know that by 1986-87 when I was shopping for a new set they were definitely listed in the price lists as "USA Custom" which at that time was the only line Gretsch had other than the abysmal imported "Blackhawk" line of that era which were just stencil kits. The USA Custom line remains the link to the earlier round badge and stop sign badged drums, more or less following the same "recipe" as those classic drums.