The first class to receive the update is the Medic, who can earn three new weapons -- Blutsauger, Kritzkrieg and Ubersaw -- obtained by earning 12, 24 or all 36 new medic achievements, respectively. All the new achievements, including a complete Medic icon set (PNG files) can be found at the Gold Rush site.
Update: It's also worth noting that this weekend, May 2 - 4, Steam users can play Team Fortress 2 for free. That includes the Gold Rush update. You can pre-load the game now.