The GMB is one of the UK's largest trade unions. If they don't have teeth, nobody does, so I'm more than cautiously optimistic. This is excellent news.
Supporting Owen Smith (an anodyne career politician who is now a PR flack) in the Labour leadership election in what looks like a rigged vote isn't indicative of a pro worker outlook, either:
https://voxpoliticalonline.com/2016/10/17/who-do-you-trust-w...