Can the deal 6 damage triggered ability from Riptide Rex be reacted to?

Ruling: Yes, the deal 6 damage can be reacted to. While permanents (units and gear) themselves cannot be reacted to because they don't linger on the chain, play effects are triggered abilities that enter the chain separately and can be reacted to.

Sequence:

• You play Riptide Rex by paying its cost
• It enters the chain but resolves immediately (no reaction window)
• Its play effect (deal 6) triggers and enters the chain
• Here each player can react with reactions

Nuances:

• Permanents themselves don't linger on the chain and cannot be reacted to
• Play effects are triggered abilities that are put on the chain as pending items after the permanent enters the board
• The distinction is between the permanent itself (can't be reacted to) and its triggered ability (can be reacted to)