I’m thinking about building a ranger who worships wind spirits, and can lead them in battle so I’m thinking this would be my swarm, but I was wondering if I could multi class into warlock.

Would my Gathered swarm feature apply to my eldritch blast, like move the creature 15ft, or extra 1d6 pierce?

And then would that work on top of repelling blast if it does?

Edritch blast is a spell attack so you should be able to add the swarm affect to the attack.

Eldritch Blast would work, yes. Gathered Swarm only requires you to hit a target with an attack. Since it does not specify that the attack must be a weapon attack, spell damage works. As for Repelling Blast and the push option from Gathered Swarm...not sure on that one.

Unless the DM says so,
- Yes, eldritch blast is an attack and would apply the gathered swarm effect (it applies to any attack, even bow attacks).
- It could apply both effects, you stack them any way you choose and resolve them in order, separately:
  * Repelling blast will always be 10ft away.
  * Gathered swarm needs a Saving Throw, and move any way horizontally.
  * they both are forced movement, so they don't generate attacks of opportunity.

Hope this helps