That website has made an incorrect link to GISC.
Supporters Trusts are usually "Mutuals" (co-operatives) - presumably why it is listed on that site.
GST was founded (IIRC) in 2003 - long after (decades ?) GISC.
This was after "ITV Digital", when GFC was building towards its £9,000,000 debt - so it seemed there might be a risk of insolvency/liquidation.
And around the time that AFC Wimbledon arose from their own Trust.
GISC is (AFAIK) unregulated.
(other than its own constitution)
Whereas most Supporters Trusts are regulated, registered entities - so provide a sort of insurance around which fans can "save" their Club if the worst should happen.
That said, it would be remarkable if some fans were not members of both GISC and GST.
In happier times, Trusts are one of the vehicles that the EFL has in mind for fan representation on Club Boards.....
...if the owner let's them !
https://thefsa.org.uk/about/affiliates-and-associates/#LEAGUE-ONE
Scroll down and see GST and GISC listed separately as affiliates.
Presumably before the merger between the Football Supporters Federation and Supporters Direct, GISC and GST were each members of either FSF or SD.