How to remove the excessive saltiness from gammon?
Soak at least overnight. In addition to that, consider a sweet glaze like apricot, or an acidic one like one that includes cider vinegar. Best yet might be all three, an overnight soak (change the water a few times) and a sweet, acidic glaze.

If you still find it too salty, go ahead and try boiling briefly in fresh water (blanching) after soaking, and then plunging in ice water before you continue to glaze and roast. If all of that isn't enough, and you are choosing the lowest salt gammon available to you, then gammon isn't your thing. Try a fresh ham instead.