I agree with Metalhead on the thickness. I'm going to assume you're part of a HEMA group from what you're describing. From my experience those guys don't hit anywhere near as hard as SCA heavy fighters do, but the metal swords (if you guys are going that route rather than wood/plastic) put out much more concentrated pressure.
You could probably get away with 18 gauge mild, but you will be hammering dents out of it on a regular basis. I personally would not do it though, it's not worth the risk, particularly over your thumbs. If you're going to go with mild steel 16 gauge is what I would recommend. 18 gauge SS or heat treated carbon steel would probably be fine as well.
If you're going to be doing any polearm work like you mentioned use clamshell gauntlets rather than fingered. They're both easier to make, and will give better protection. Make sure that whatever design and material you use wraps around the sides of your fingers so that if your hand is struck the plates bottom against the haft of the weapon, not against your hand.
As someone who had to give up fighting following an injury I urge you to be careful. I fought for years with waxed leather demi-gauntlets because they passed the minimum requirements and I could make them myself for cheap. And then while having some torn ligaments in my wrist cleaned up last fall the surgeon also decided I needed some bone scraped out of my hand because it had softened due to "repeated trauma" as a result of sword fighting.