What exactly do you want to de-restrict?
The info Jam35 posted is only relevant if you want to de restrict the boost, I don't think 1st and 2nd gear ar restricted to low boost in the GT Turbos.
If you want to de-restrict the speed limiter, you need something like a HKS SLD (speed limit defencer) or you could buy an uprated plug and play ECU or a stand alone ECU.
Another option which is a lot cheaper is to buy a speedo converter, that converts KMH into MPH, you would need to change the dials as well but it just plugs into the gearbox I think and that will delimit the top speed.