Gisa, Glorious Resurrector

• WWizards of the Coast2021-09-24
  Official Ruling
  Once a creature with decayed attacks, it will be sacrificed at end of combat, even if it no longer has decayed at that time.
• WWizards of the Coast2021-09-24
  Official Ruling
  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.
• WWizards of the Coast2021-09-24
  Official Ruling
  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."
• WWizards of the Coast2021-09-24
  Official Ruling
  Decayed does not create any attacking requirements. You may choose not to attack with a creature that has decayed.
• WWizards of the Coast2021-09-24
  Official Ruling
  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.
• WWizards of the Coast2021-09-24
  Official Ruling
  Decayed does not grant haste. Creatures with decayed that enter the battlefield during your turn may not attack until your next turn.
• WWizards of the Coast2021-09-24
  Official Ruling
  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.

Rulings via Scryfall