After Nevio Zeccara drew the first two issues of Gold Key's Star Trek, Alberto Giolitti took over the art. The studio artists who helped him include Massimo Belardinelli, Giorgio Cambiotti, Mario Pedrazzi, Giovanni Ticci, and Angelo Todaro. I can't tell how much the studio artists contributed, although I think I see different finishes on different issues; the inks on #28 are pretty sparse. Saying that Giolitti pencilled a run of issues and Ticci inked them seems oversimplification to me.
The GCD's attributions of some Giolitti issues' inks to Sal Trapani or Alden McWilliams I'd call mistaken. McWilliams took over as regular artist with #40, after doing #38.
The first nine issues used photo covers; then George Wilson provided paintings for most of the rest, with a number of exceptions. #60 and 61 were line drawings by Mike Roy, and working backwards with an eye on his style (such as in Kirk's face), I'd credit him with the painting on #59, and possibly the cover paintings on #47, 48, and 58. There are covers by other artists as well.
#39 is the issue with the forgotten artist. It contains a mixture of his art and the Giolitti studio's, but I think this page flashing back to the Twentieth Century is all his pencils.
The officer in the first panel and the girl on the left of the third are the most obviously José Delbo's.
As far as I can tell, Delbo in the U.S.A. drew everything but the Enterprise crew figures; Giolitti and company in Italy supplied most of those (and all the inks?). On this next page, Spock and Kirk are Giolitti's, but the other figures are, I think, Delbo's pencils.
I may be off on just how the work was divided, but definitely Delbo contributed to Star Trek 39.
In #36 as well, I think I see some Delbo figures here and there, but they're nowhere near as obvious as in 39.
#39's story was written by Arnold Drake; I'll list the uncredited GK ST writers next week.