In addition to a full new update that includes a new official CP map, a bunch of new community maps, new taunts and new acheivements, today's Gun Mettle Update includes a paid campaign. Buying a $5.99 Campaign Pass gets you access to weekly challenges that allow you to earn new weapons. A portion of your campaign past cost goes towards community map makers, so you can rest easy knowing your cash is going somewhere good.
Learn more about the update here.