Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.

Because creatures controlled by opponents aren't dying due to Gisa's replacement effect, any "when [this creature] dies" triggered abilities those creatures have won't trigger.

Decayed represents a static ability and a triggered ability. "Decayed" means "This creature can't block" and "When this creature attacks, sacrifice it at end of combat."

Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.

Gisa does not grant haste to the creatures that are returned to the battlefield, so you normally won't be able to attack with them the turn they are returned.

Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.

If a creature your opponent controls would die and more than one effect would cause it to be exiled, that opponent chooses which one to apply. If the creature is exiled due to some other replacement effect, it will not be returned to the battlefield with Gisa.