If you're needing to stretch leather and not just form it, then you need to dampen it with alcohol. It will let the fibers in the leather stretch and stay at the size you want (within reason). Garment grade or boot grade leather will just return to its normal shape after it dries when wetted with water.
Also be sure to treat it with something like Skidmores Leather Cream. The alcohol will knock any of the oil in the leather out and the cream will keep the gloves from cracking. You should be able to find Skidmores at most tack shops (saddle shops) You can treat your leather jackets with it, saddles, chaps, boots...basically any leather with a smooth finish but keep in mind that it will darken stuff up for a while. ABSOLUTELY don't use it on suede (or elk
) Take it from a (former) saddle shop guy, its good #$%*