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Why can't I use "GMAN" as my tag in halo 5?

OP Garthski

In previous Halo games I've always been able to use GMAN as my service tag but now in Halo 5 I cannot use it and am told it is an "Invalid Entry", why can't I use it all of a sudden in this game?
Honestly have no idea, my last name is Dunn, and it says the same thing.

What world is that a bad or derogatory word?