1. Most of the technical work on version 0.1 of having a working system.
2. Most of the ideological work
3. Most of the legal work (Linux is GPL too)
They're the original "inventor."
Linus finished building it, but without Linux, Herd would still eventually be ready, and we'd have the same thing, only a couple years later. I'm not trying to downplay this contribution; Linus and Linux are pretty special too. However, Linux wouldn't have happened without GNU; Linus could not have created all of that.
Many people have since improved upon it. Like the Ship of Theseus, their original work is a tiny portion of the system in 2022. However, without them, the whole system would not exist. If that didn't happen, even things like Android, many iterations later, couldn't exist, at least in anything in their present form. "Open source" wouldn't exist either (which was created as a counter to "free software").
It's hard to overstate this contribution.