How to make a spawn animation (geckolib)

You could use the "entity joins the world" global trigger and play your animation for that

The problem with using Global Triggers like "Entity Spawns" is that by default entities take the idle animation when spawning, since they're in idle mode bc they spawn standing still, this is a problem because the first frame of the idle anim always is played, and then immediately switches to the spawn animation, so this method works but it doesn't look as good as it could

Create a spawn animation for the idle animation of a entity, then replace the entity with a desire one with the same model as the previously entity, but with a properly idle animation