Is it possible to cast reaction spells 'between' repeated damage spells to try and save the unit? I.e. playing gust on my 2 hp unit when my opponent plays repeated bellows breath?

Yes, you can use Gust to react to Bellows Breath in a way that saves your unit, but there is an important distinction regarding the "repeated" nature of the spell.

The Ruling

You can play Gust as a reaction to Bellows Breath on the chain. Because Bellows Breath is a single spell (even if the [Repeat] cost is paid), you are reacting to the spell itself, not the individual instances of damage.

However, once Bellows Breath is played and the Repeat cost is paid, the spell is committed to its targets. If you use Gust to return your unit to your hand before Bellows Breath resolves, your unit is no longer on the board to be a target. According to Rule 359.3.e.5, when Bellows Breath resolves, it will simply ignore the now-invalid target (your unit), and the damage will not be dealt to it.

Why you cannot react "between" the repeats

It is important to understand that Bellows Breath does not create multiple distinct instances of a spell on the chain.

• Paying the Repeat cost means the instructions of the spell will be executed twice upon resolution.
• The spell exists as a single entity on the chain.
• You cannot play a reaction spell "between" the first execution and the second execution of a single spell's effects. You must react to the spell while it is on the chain, which forces the spell to resolve as a whole (or fail to resolve its instructions) afterward.

Summary of the Interaction

1. Opponent plays Bellows Breath (paying the Repeat cost). It goes on the chain.
2. State is now CLOSED.
3. You play Gust (as a [Reaction]) targeting your unit. It goes on the chain on top of Bellows Breath.
4. Chain resolves LIFO (Last-In, First-Out):
   • Gust resolves: Your unit is returned to your hand.
   • Bellows Breath resolves: It attempts to deal damage to your unit. Since your unit is no longer on the board (it is in your hand), it is an invalid target. The instruction to deal damage to that unit is ignored (Rule 359.3.e.5).

Result: Your unit is saved, and it takes no damage.