If you’re in the UK, most gammon joints, even smoked ones, are now quite mild. I never soak mine. However, if you’re concerned about food being too salty, or if that’s not to your taste, you can soak it in fresh cold water for a couple of hours before adding it to the slow cooker. If you’re in the USA, uncooked ham joints tend to be much stronger in flavour so follow the instructions on that joint about whether you should soak it first.