what could be causing the gunk that keep s clogging the radiator and thermastat
It could be a couple things. It could be engine oil that is getting into the cooling system from the head gasket being blown - in which case you need to also check your engine oil to see if it has been contaminated. If so, it can cause severe engine damage if you continue to operate it.
It could also be transmission fluid that has gotten into the system from the transmission cooler leaking into the radiator. This will sometimes also cause the transmission fluid to get contaminated and cause transmission damage.
Something along this line may have been the original cause of the overheating in the first place. Then a blown head gasket may have occurred as a result of the overheating (the aluminum heads on those engines are very unforgiving when it comes to being overheated).
Honestly, I recommend getting a professional evaluation of the situation to avoid misdiagnoses, which could turn out to be extremely expensive. It would be much cheaper to at least get a professional to check it out and tell you the full extent of the damage so you know exactly what you are facing here.....then if you decide to perform the actual repairs yourself, you can possibly save a bunch of money that way.