GLTFLoader uses the default three.js animation system (COLLADA, notably, does not). So with glTF 2.0, you can play your animations like this:
let mixer;
const loader = new THREE.GLTFLoader();
loader.load( 'model.gltf', ( gltf ) => {
const model = gltf.scene;
mixer = new THREE.AnimationMixer( model );
gltf.animations.forEach(( clip ) => {
mixer.clipAction(clip).play();
});
});
Finally, call mixer.update(deltaTimeInSeconds)
before each frame.