I worked in the leather goods industry for about ten years. Leather needs to be moisturized periodically or it will degrade, crack, and basically turn to dust like our skin does. An over the counter hand moisturizer is actually better than nothing for that. Will the moisturizer somewhat compromise the waterproof barrier? Yes, a little. But, without it your gloves will wear out a lot faster. It is possible to moisturize then and add a coat of waterproofing treatment afterwards a couple times a season if you use them a lot. The stuff you spray on tent flies would probably work and also NeverWet treatment would probably be good. Not certain that those chemicals wouldn't also compromise the longevity and durability of the gloves making it more brittle and less supple though.
In the end, gloves wear out. If I paid over fifty bucks for a pair of gloves I'd take supple and less likely to crack for an additional year or two of life over absolutely 100% waterproof any day. Besides, on the wet snow days where water and sopping wet gloves are an issue yo don't need something super warm. On those super cold sub zero days you don't really need anything totally waterproof. So, the good gloves for really cold days are going to get leather care lotion periodically. The old ones will get NeverWet or waterproof spray for spring days.
But, my really heavy gloves aren't leather at all, but I have a couple of full grain leather jackets including a full length duster. That one sucks in a LOT of leather care hahaha. Also, the oilyness of the lotion does help it bead water pretty well. It's more waterproof conditioned than it is dry. I'd think the same would be true for gloves.
The best leather care I've used is Neatsfoot Oil