Am I to understand that this is a fare that travel agents, for example, can use, but is unavailable to the general public?

It certainly appears that way given the "Travel Industry" notation, and the fact that I can't find a published UA "G" fare on any of several booking engines and other services that I subscribe to.

So if you're using an essentially unbookable, unpublished fare that the general public cannot use, then I think it's fair to call that a "cheat."

On the other hand, if you can show me how to book one...