See docs for the animation system and THREE.AnimationAction. You’ll probably write something like this:
var model = gltf.scene;
var mixer = new THREE.AnimationMixer( model );
var action = mixer.clipAction( gltf.animations[0] );
// Play one animation
action.play();
// Play all animations
gltf.animations.forEach( function ( clip ) {
mixer.clipAction( clip ).play();
} );