Thanks that worked,
here the amended code for anyone finding this post:
utils.loadGlbContainerFromAsset(this.glbAsset, null, this.glbAsset.name, function (err, asset) {
var renderRootEntity = asset.resource.instantiateRenderEntity();
self.entity.addChild(renderRootEntity);
// add animation
renderRootEntity.addComponent('anim', { activate: true });
renderRootEntity.anim.assignAnimation('clip', asset.resource.animations[0].resource);
renderRootEntity.anim.baseLayer.transition("clip");
});