Gleeful Arsonist

• WOTC Official Reminder Textofficial rules
  Undying — When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.

• WWizards of the Coast2024-09-20
  Official Ruling
  If Gleeful Arsonist is no longer on the battlefield when its first ability resolves, use its power as it last existed on the battlefield to determine how much damage it deals.
• WWizards of the Coast2024-09-20
  Official Ruling
  Gleeful Arsonist's first ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
• WWizards of the Coast2024-09-20
  Official Ruling
  Once undying returns the creature, it's considered a new object with no relation to the object that it was. Auras that were attached to it won't return to the battlefield. Equipment that was attached to it will remain unattached. Any counters that were on it won't be put on the new creature.

Rulings via Scryfall