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Special abilities default to play unless they specify otherwise (or have a default condition specifying otherwise such as banding). Gates doesn't specify otherwise so the demon added to battle must come from play. While it might end up being a band ability if there is already an EC in battle, add to battle is a distinct, separate ability (as far as I'm aware) with different default conditions.
I would like to at least keep banding from hand a legal play. I believe there's a great deal of strategy in being able to band in a "surprise" character from your hand.
Any ability that adds a character to a side of the battle where at least one other character is present is a band ability.
Default Conditions Targets must be in a territory or in holder's hand.