The 1st knife produced by Spyderco was the "Worker," which according to SpydieWiki, 1st used Gin-1 (in 1981) and later ATS-55 ("around 1997-1998").
See:
SpydieWiki on the Worker
SpydieWiki also reports that ATS-55 was 1st used in the Mariner (the 2nd knife produced by Spyderco) in 1997. So, it would be likely that ATS-55 was 1st used by Spyderco in the Worker in that year too.
See:
SpydieWiki on the Mariner
A quick scan of the build history of other early Spyderco models does not show any earlier use of either steel in any other knives.
From what I saw it looks like Spyderco stopped using Gin-1/2 when it started using ATS-55 in 1997-98 and the use of both steels overlapped the use of AUS-6/8 & ATS-34.
Looks like the use of ATS-55 was short lived because I didn't notice any reference to it after 2001. After that, VG10 was Spyderco's go-to steel for most knives and remains a steel they still use to this day along w/the many other "modern" steels created since.
So, just based on what I saw, Spyderco used Gin-1/2 for about 16-17 years between 1981-1997/8, only used ATS-55 for about 4 years between 1997-2001 and has been using VG10 for about 19 years between 2002-now.
I did NOT do a thorough review of this data. So, anyone else who sees any gaps/ommisions is welcome to make any addenda/corrections.