Gammon can be incredibly salty. Boiling it helps to get rid of some of the salt. So, even if you intend to put it in the oven, boiling it first would still be wisest. (I like Melv16's idea).
I read somewhere that if you are worried about it being salty you should cut a sliver off it and fry it to test it. I never bother personally. I roast mine and then make a glaze from brown sugar and cinnamon.
melv that is what I always do. Boil with bay leaves, celery, carrot, de-rind and diamond cut the remaining fat, stud with cloves and honey glaze. To eat hot and some cold, just cut in half when taken out of the oven. SOAK OVERNIGHT first. As buen says it can come up salty.
Togos method is my favoured choice, but rind of and cover with your preferred choice of coating I.e honey, brown sugar, and some cloves, prior to baking. Hope you have a large oven with meat,potatoes, stuffing and parsnips to cook etc. If you need to remove gammon if you need the space, I would slice what you need cover and microwave. Sorry if I teaching my granny to suck eggs
Bear with me on this. I always reinforce the stringing and never trust the flimsy pink stocking things that they now sell gammon joints in. I have a butchers needle and do my own re-stringing. When you bring to the boil it always blows the joint out of shape. If you have no needle just make sure that you tie the blighter up well. Once up to boiling cover, simmer, add more BOILING water as you go.
If you have an unsmoked joint there is not really any need to soak overnight. I have boiled in cola and ginger beer over the years - both nice. Then finished in the oven with a glaze of coarse grained mustard and honey. One tip, I use a disposable foil tray for this, saves the trauma of washing up what can be a very messy roasting tin!