There's absolutely no need to require a three-man team for the Gorgon Maze, or require a four-man team for Atheon. Why choose to maintain this now, when the content is no longer relevant to endgame? Duo'ing the first three parts is, without doubt, some of the most fun possible in the game's single player campaign. Why restrict the rest? It's a dumb, dumb decision, and it needs to be removed, if the content isn't going to be brought up to date.
Edit: with regards to the Gorgon Maze - the passage was blocked today *after* spawning in with three, and reducing numbers, (sherpa on a third, his first run.) This has never happened to us before - we were always able to go in with 3+, and then disband to solo run the Maze and the jumping puzzle before reforming.
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You can solo the Gorgons. I do it all the time as people always get caught (somehow) and it's faster that way. Gatekeepers require two, as one person has to hold the portal open. Atheon requires three, and two people get teleported. (I think)