I'm pleased to report that the experiment to vacuum seal Gamblin mediums proved successful. At least partially so. I used the Vacu Vin Wine Saver to easily seal the 16 ounce Gamblin bottle and it worked perfectly. However the stopper is too small for the 32 ounce bottle and too big for the 8 ounce bottle. So I'm still using rubber stoppers for those sizes.
This model of vacuum stopper makes a small click when a vacuum is achieved and I was surprised how much pumping I did to extract the air. But once you know how long it takes (just a few seconds, really) it's a snap to seal. I recommend getting the model with the extra stoppers (see link below).
Thanks to fellow painters Dave Casey and Robert P. Britton, Jr. for this terrific idea! I'm sure it will save hundreds of dollars of medium over the years.
Brad Teare October 2013