Hi Lenny -
Sorry to hear you're having problems with your new horse.
I really don't think it's GBD. Typically, gas bubble emboli on the tail are clear bubbles. These are clearly white. That's not gas in there. There are a variety of things it could be and unfortunately you can't be positive about a diagnosis like this without a culture or biopsy. The bad news is that many of the things it *could* be aren't readily curable.....mycobacterium, nocardia, glugea. All of these diseases can present with infected nodules on the tail. It doesn't look like lymphocystis to me, which can be benign, but is usually more like a cauliflower shape. It is possible that it is a simple case of vibrio infection which is serious, but treatable. In the absence of other valuable suggestions I'd treat for vibrio and hope. You'll need to treat in a hospital tank of 10 gallons. Add 250 mg of Neomycin for each gallon....2500 mg total. It is also beneficial to treat concurrently with Triple Sulpha per package instructions. Use no bio-filter in the hospital tank. These medications are bio-filter toxic so it will only contribute to your ammonia issues to include a bio-filter. You'll have to control ammonia with sizeable water changes each day replacing the medication in proportion to how many gallons are changed. Even if there is no improvement you MUST continue the treatment for a full 10 days or you risk breeding a resistant bacterial strain that will be introduced back to the display tank.
Because of the other possible causes of these nodules I would watch the other horses VERY closely. It wouldn't be overly cautious to remove this horse altogether and isolate him away from the herd.