well, it says specifically that free spirits with both powers can force anyone into astral combat, stating it can't suck unwilling victims into astral space. So firstly, it isn't actually pulling the astral body out of the body; it's more like astral perception. Secondly, although it requires an exclusive complex action to activate the gateway power, it also requires an Exclusive Simple action to activate the Animal Form power - this doesn't mean that the act of being an animal is exclusive; only the changing of forms is. Therefore, it stands to reason that maintaining the astral gateway isn't an exclusive action, although opening it is. Same goes for Human Form - of course, these violate the Critters rulebook description of exclusive action powers.
But, even if astral gateway is an exclusive action to sustain, that doesn't mean that the spirit cannot perform astral combat as long as this doesn't use any of the spirit's other powers. After all, cannot a magician sustaining an exclusive spell also fire a gun or fight?