Fun Fact: The G-Man was, at one point, involved with Portal 2's story, based on developer comments in The Final Hours of Portal 2 and leftover map logic. sp_a2_bridge_intro, the map introducing Hard Light Bridges, has map logic remaining in it referring to a cut G-Man encounter. I can't 100% be sure when he was cut from the story, so it's up in the air if he was cut early or late into the Multicore era of Portal 2's development.
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well maybe the Gman is watching Chell and we just can't see him at all