For cleaning, if you have a pressure washer (preferably a hot water model) knock off the built-up hardened grease/dirt. Follow that with a strong detergent (Purple Power, Simple Green, Zep, or Castrol Super Clean) or a degreaser (like Stoddard Solvent, kerosene, or diesel). Follow this with thinned detergent in hot water. For the internals, use the solvent, since you don't want water-based detergents in there when you fill it with fresh grease.
These were not made for complete disassembly. If it leaks a little around the pump shaft, that isn't unusual. If it leaks a lot, most people get another gun. All of these guns will seep around the grease reservoir if they sit around. It is not uncommon for them to leak around the rubber seal that pushes the grease up the disposable tube. It should not leak around any threaded joint, and if tightening (or adding teflon tape and tightening) does not fit it, replace the tube/hose. If that doesn't fix it, you could try retapping the female pipe thread.
These guns were considered expendable, and often discarded when they started leaking. You have one of the better ones, but it still wasn't designed to last forever.
