Toyota diesel engine gaskets to different places are made of metal with some coat of rubber like surface.
Do they need or do you use any sealant when assembling engine or is ok with just the gasket as it is.

 

I would think that the rubber part is meant to seal, so I don't know why you'd also need sealant on them as well? I would strongly advise that if you do not already have one, get the factory manual for that specific engine & see what it says. In any place that specifies the use of sealant, I recommend Toyota FIPG or Permatex Ultra Gray or Ultra Black.

I don't know too much about the diesels but on 20R & 22R gas engines most of the gaskets do not require sealant. They just need to be installed in the correct orientation and with the correct torque to the fasteners. I just installed a new OEM style cork-rubber oil pan gasket on my 20R with no sealant used & it's bone dry with no oil leaks.