From 1980 through the early '90's, the nut material was Plastic. Reference:
Greg Gagliano's G&L webpages.
Also, see this post:
Was Brass nut ever offered as original?.
In the late '80s, the steel Wilkerson Roller Bearing Nut was an option for all instruments. Reference:
Album: January 1, 1988 Pricelist.
Also, this same Pricelist indicates that the SC-3 came with a Bone nut. However, I have never seen any SC-3s or any listings of them indicating that any of them had factory installed Bone nuts.
When G&L first offered the Graph-Tech graphite nut it was the stock nut on the Legacy Special in 1993. Reference:
Album: 1993-94 Catalog.
At the time the stock nut for most of the other guitars was Micarta ("High-Tech" Plastic). Later,
the graphite nut became the stock nut for the Comanche and S-500 models and an option for the
other models.
In September 2009, G&L switched from the Micarta nut to a 100% Natural Bone nut on all models, including the Comanche,
the Legacy Special and the S-500. The Graph-Tech nut remains listed as an option for all instruments. Reference:
Correction on nut material used by G&L.
NOTE: This information was researched by me using the reference sources indicated. If anyone has any additional information which can add to or correct this information please contact me with details.