Based on the internet resources I have found (pics, vintage lure disc boards, catalog page pics, etc) it looks like Gibbs only made the following production GTS models:
Saw these in a catalog page...
* GTS-1 1/2
* GTS-2
* GTS-3
* GTS-3 3/4
and
Saw this one in forum pics...
* GTS-7/8
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