105 GXL Landcruiser auto 1HZ with aftermarket Denco Turbo kit

Advice?..........Spell check..............

Gday Hercules S...

It's a bit hard to read your post!

My vote... Start with a replacement engine....

I can't see how the compression pressure would find it's way to put water into the gearbox even though the gearbox cooling heat exchanger is in the bottom of the radiator. Never heard of than before.

Once you have the head gasket problem sorted it should be ok to use into the future if all mechanicals and the head and block are good. If you don't punish it and use it properly then all will be ok.
It isn't the heat so much as the extra combustion pressure the turbo creates that is the problem.

Been there done that in regards to a 1HZ with A/M turbo, I now own a totally stocked engine 1FZ FE Petrol.
If you want long term reliability from a turbo'd 1HZ, you have to keep EGT's(exhaust gas temperature) as low as possible. To do this you NEED an intercooler, you need your pump and injectors in A1 condition, you need your cooling system in 100% working order, your pump timing set correctly and fuel mixture correct. Some may also say a boost compensator although I never did and has great temps.
Personally, I wouldnt go down that path again, go for a 1HD FTE conversion, much better engine thats designed for a turbo and relatively easy conversion these days. And by he time you rebuild your engine your well on the way to your conversion.
And a turbo increases your temps., a intercooler will help decrease them.

What you need to do is fit an Exhaust Gas Temperature (EGT) gauge so you know how much heat the engine is generating.
There is no heat resistant head gasket or kits,the only thing you can do is avoid generating the heat by watching the gauge and easing off when its gets too hot.
The probe or sensor for the gauge is fitted by tapping a hole into the exhaust manifold

Usually its the pistons that aftermarket turbos are most likely to damage on a 1HZ,they are too thin to cope with the heat a turbo can make and unfortunately ,the companies that sell these aftermarket turbos are a bit quiet on the issue.
There are turbo pistons for the 1HZ available from a company in South Australia called Engines Australia that are thicker and heavier than the stock pistons.

You may also want to see a diesel injection shop about getting your fuel pump checked to see if its overfuelling,this can also cause excess heat.