Cleaning leather bike gloves
It doesn't matter what it's made out of, bike stuff gets stinky. Especially the stuff that goes on your hands and your feet since they're similar appendages that you keep similarly wrapped up and pressed against surfaces that don't breathe well.

There are a lot of active wear specific detergents out there- Sportwash, Sport Suds, WIN, Nikwax, etc etc. I've tried none of them, but probably should. Most of these detergents are supposed to be especially well suited at removing odors from synthetics. I see a lot of folks recommending castile soap or oil soap for leathers. This is usually for larger pieces like jackets and handbags. Given your situation, I think you're probably overthinking it. A soak in warm soapy water using a bar soap that you'd use in the shower followed by a brisk hand scrub and rinse should be sufficient. Air dry them, and if you're really worried, use a light leather conditioner once they've dried.

They're going to get stinky again, and quickly. There's just no way around that. Your hands are just funny looking feet at the end of your wrists that you're pulling these leather socks over and perspiring heavily into. Wash them as necessary with mild detergents and realize that- just as with almost everything else you purchase for the sport of cycling- the gloves are consumables and they won't last forever.