Case of the gateway express not being triggered by mobilize or other comes into play attacking abilities.
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Rezzahan#77802
commented
To attack with a creature, it has tzo be declared as attacker in the declare attackers turn based action. No creature that enters attacking is declared as attacker, and thus never attacked.
508.4. If a creature is put onto the battlefield attacking, its controller chooses which defending player,
planeswalker a defending player controls, or battle a defending player protects it’s attacking as it
enters the battlefield (unless the effect that put it onto the battlefield specifies what it’s attacking).
Similarly, if an effect states that a creature is attacking, its controller chooses which defending
player, planeswalker a defending player controls, or battle a defending player protects it’s attacking
(unless the effect has already specified). Such creatures are “attacking” but, for the purposes of
trigger events and effects, they never “attacked.”