What do you need to do to change the head gasket of a rb20det Nissan skyline?

Time and money.

Head gasket set around $200

Thermostat same as VG30dett $15-30

New head bolts ARP is the best

Have the head checked for cracks or warpped and they can resurface the head usually $100 or so

Get a coolant system flush.

Might also change timing belt while you have the head off along with the tensioner and spark plugs.

It would not hurt to change water pump as well.

Would be a great time to upgrade to a larger radiator as well if you get on your rb hard they seem to make a lot of heat.

Well I think I covered everything as I have to do this to mine as well do to my little drifting escapade.