It's not quite as effective as xanthan gum but I find arrowroot flour works wonders. You can also try locust bean gum or psyllium husk (soak it in water before mixing so that it can dissolve)
Those are more starchey than the gums and are meant to be mixed with amaranth, rice, quinoa, or other more coarse flours it gives you that same "slimey" start that hold the bubbles as the bread cooks. That's generally gone by the time it's cooked.
When you're using guar gum, try to keep the quantity down to about 1% by weight of the dry ingredients. Also give it time to fully hydrate before adding the rest of the ingredients. I normally mix mine in dry with the flour and then add milk or water to get things wet and then leave it for a couple minutes before continuing.
If the gum (or even the arrowroot flour) is properly hydrated it will be more effective in baking. Plus you can use less liquid overall which is good because it's easy to overdo the liquid in GF bread.
Saltpeter is potassium nitrate, which does not directly cure meats. Bacteria convert nitrate into nitrite, which is the real preservative. Saltpeter can be replaced by a smaller amount of nitrite to get the same curing effect (most commercial cured meats do this), though a prolonged cure that converts nitrate into nitrite can develop more flavor.
Tender Quick is not a direct substitute because it contains mostly salt. I've heard that you can replace the salt in your recipe with Tender Quick, and drop the saltpeter, and have a success. You would have better luck finding a recipe that was meant to use Tender Quick, though.
It is definitely possible to buy (food-grade!) saltpeter. I would check online, or at specialty stores. It's a little more difficult than picking it up at your local grocery store, of course.
(Chemistry lesson, courtesy of McGee: nitrate (NO3) is converted to nitrite (NO2), which then reacts to form nitric oxide (NO), which bonds to myoglobin in the meat, which turns it pink and prevents oxidation. Nitric oxide is also present in smoke, which gives that "pink ring" around the outside of smoked meats.)
Best Answer
Your closest substitute will be pineapple, either juice or puree, which is much more widely available worldwide than green papaya.
Papaya contains a natural tenderizer called papain; pineapple contains a different one called bromelain. While the actions of the two fruit-based tenderizers won't be exactly the same, they will be very similar. Plus both will impart a tropical-fruit-flavor.
Your other alternative is to find dried paypaya powder. While still hard to find in many places, you could order it over the internet, and it would keep for a long time. The enzyme Papain survives drying*, and thus can still be used in powder form, although you'd have to experiment with quantities.
If none of that suits, see this answer for other ideas.
(* there are multiple references to this but they're all spammy, so no links included)