Mines, trust and possibly others make a direct replacement for the gtr boost gauge. Its a matter removing the entire cluster. You'll need to disconnect all wiring and hoses that are connected to the cluster, take the backing and then the stock boost gauge out. Put the aftermarket one in and reconnect the hoses and wiring.
If its not a direct replacement I'd go with other guys suggestion and put in the pillar.
Also the direct replacements are expensive and a second hand one is hard to find, took me ages to find one.
hope that helps
cheers