They're known as NPT (National Pipe Thread). It's a tapered thread so it's less likely to leak OR back out. Yes, TIGHT.
Use an acetyline torch (propane or butane is not "hot" enough). Heat the plug (NOT the block) until it glows. Allow it to "cool" just until the glow disappears. Hit it with some parafin (canning wax works really well). Use a 1/4" drive ratchet to remove them. If you get to them while they're still cooling (hot to the touch, so don't burn yourself), they'll come right out.
If for some reason, you DO "round" one, drill all the way though the plug with a 3/8" drill. Using a LARGE "EZ-Out", heat it again, hit it with the wax again and insert the EZ-Out and it should come out much easier.
BTW, they are "galleries", as in a shooting gallery or art gallery. A galley is a kitchen on a ship... (:-