I'm not quite sure what you mean. Animating Gideon on an opponent turn does not mean it can attack, just like how animating a Mutavault on your opponent's turn doesn't mean it can attack.
If you mean that your opponent is attacking Gideon in combat, then you make Gideon a creature, Gideon will only deal damage if it blocks something. Gideon won't deal damage to creatures that are attacking it, only if Gideon is declared as a blocker at the beginning of the declare blockers step. For instance, if your opponent attacks Gideon with three 2/2 creatures and you use Gideon's +0 to make him into a creature, he can block one of the 2/2s, and Gideon will deal damage to that 2/2. But Gideon won't deal any damage to the other 2/2s, since Gideon is not blocking them (and since Gideon can't be dealt damage that turn, the creatures won't deal any damage to Gideon).
Nathan Long
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