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.

When soaking very salty meats it is good practice to change the soaking water frequently. For the first 2 or 3 hours change hourly, and after that increase the time in between changes to 2 or 3 hours.

I have also found that a few carefully placed slits in the thickest parts of the meat can be helpful. You would want the slits to be no more than 1" wide, but they should go deep into the meat even to the point of reaching the bone. This will allow the water to get deeper into the meat.

Heavily salted meats, like gammon, speck, country ham, proscuito di parma and such aren't intended to be eaten as a steak or roast.

They're typically used in small amounts as flavoring in other dishes.

If you're looking to roast a ham, you'll want to find a different variety that isn't as heavily salted. There are 'fresh hams' which isn't cured at all, and 'city hams' which are cured, but still need refrigeration for long-term storage.

To remove salt from Haddock, I use milk. I think it would be a good idea to let it soak for 6 hours. Then 1 hour in water. The meat will be softer anyway.