Wear a clean pair of boxer's hand wraps to protect your hands and wrists and absorb sweat. Not wearing hand wraps with your gloves is like not wearing socks in the shoes you train in everyday, it is bound to get sweaty and gross. Wearing boxing wraps is not only more sanitary, it also has the added benefit of keeping your wrist and hand secure to avoid unnecessary injury. Again, just like your socks, make sure you wash your hand wraps after each use either by hand or in the laundry.
TIP: Put them in a delicate wash garment bag before chucking in the laundry to avoid the tangled mess.
Damp and dark environments are the breeding ground for bacteria that causes your gloves to stink. Don't leave it in your car or in your bag. After every use, wipe down the excess sweat on the inside and out to stop it from soaking further in. When you get home open up the wrist straps and air it out in a dry, well-ventilated room near a open window or in front of a fan.
Don't put it away until it's dry!
Sometimes when sweat and bacteria gets trapped in there, there's nothing you can do to fix a persistent odor. There may be no choice but to buy a new pair. When you purchase a new glove consider writing the month and year that you purchased them on the band or on the inside of the glove with a paint pen. This way you can track when you may need to replace them, often this occurs every 6-12 months but can be more often with heavy use or if you train all the time.