Glass bedding is fiberglass resin, with, or without the fibers mixed in. On a muzzle loader most only bed the breech and tang area. There are two basic advantages to glass bed. #1, The bedding gives about 99% full contact of the breech area with the barrel and tang, thus giving a full, and even distribution of recoil pressures. Much more than most rifle makers can get by carefully removing wood to make the fit. Even recoil pressures increase accuracy. In some cases, a lot, in other cases, not necessarily even measureable. #2, glass bedding does not change contact pressures with humidity or other normal weather conditions. Another advantage is that end grain in this area is sealed from oil seepage, wetness, and grime. Another small advantage is for the builder who has not the skill to inlet a good bed for the breech. This is a job worse than fitting a butt plate, for most.
To bed in a proper manner, the bedding must not show, so the bedding begins maybe 1/8" below all surface edges, and those edges still need to be inletted well for looks. The bedding, on average needs to be about 1/8" thick, or can be a tad more without problem. So, that is how much depth of wood needs to be removed. You can go forward in the barrel channel 4", to 6", or as far as just short of your first pin through the stock. It is not necessary to go up the sides of the barrel channel more than halfway of the upper flats. The barrel needs special preparation before bedding.
The very breech, and tang are going to have recesses that need to be filled with modeling clay, or you can end up with a barrel that you cannot ever remove without damage to the gun. This is super important! Fill anything that you think possible to lock the barrel in. Use common sense and check more than once. The bedding will get into even the smallest of recesses. Then the section of barrel, breech and tang must have a release agent applied, to keep it from being glued in permanent. Wax, vaseline, PAM non stick spray, or even professional release agent. Apply the bedding compound to the relieved area of wood, set the barrel in it. I have found that it helps to coat that area of the metal that you are bedding also. Doing so, seems to reduce bubbles. Especially the rear of the tang bolster, and under the tang. Either clamp the barrel in, or screw it down as normal with release agent on the screw, put the pins back in. Then clean up any runout with white vinegar. Let harden, then remove barrel. Redrill any holes in the stock that may have gotten filled. Ideally, the tang screw should have no side contact in it's hole, when you are finished. That goes for the lock screws also. If you have what you think is too many bubble holes in the bed, redo with a light coat of bedding. I will warn you. This is not rocket surgery, but if you don't have all recesses filled before bedding, you can end up in real trouble. If for some reason the barrel will not break loose, put the gun in a freezer. The metal will shrink, and if there is no mechanical lock up, it will come out. Finding a freezer long enough for a longrifle could be a problem however.
There are many brands of glass bedding compounds. Most all will do the job, but it is best to work with a type that is not real runny. If you are just going to do a breech section, most any common epoxy glue will do just as well.
There are many that will poo-poo glass bedding a muzzle loader, and I will say that if you are getting consistant, and good accuracy out of yours, and have no complaints, or problems, it probably is not worth the trouble, but if you have accuracy changes due to weather, and have done other recommended proceedures to correct it to no avail, then it is worth a try. Modern center fire bolt guns are a different animal. Bedding is a must, if you want consistent, and the best performance. If I have overlooked anything, or you have questions, just ask.
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