I was going to think up a strip, but I decided not to, as I have too few of the rules.
I agree about someone having to say "eh".
I think you have to have six different poses, but not necessarily six different characters. This assumes that all yellow-shirted Gabes are considered to be the same character. If that's not so, then I'll agree with the six different characters.
I suspect that characters who share a panel must also share a taxonomical classification (e.g. both are humans, or both are birds, or something like that -- I guess it would be okay if both were robots).
I also suspect that the strip must make mention of some kind of metal or mineral.
There must be some rule about backgrounds, because I can't imagine why else Gabe would choose such arbitrary combinations of backgrounds, but I can't figure out what it is. It may have something to do with the sequence of backgrounds rather than certain ones being legal or illegal.
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The what mentioned above is total fiction. Please don't take it seriously!