if you have a gts you should have 257mm rotors
the gts25 got 280mm x 22mm rotors which is the same as the s13 sr20det or n16 pulsar. This uses the same caliper you have, so all you need is the caliper bracket and rotor. This is a really cheap upgrade and works well with some good pads. Good thing is the rotors are dirty cheap because they are common.
Next up is r33 gts25 (non turbo). These are 280x26mm rotors, still 4 stud. They are a 2 piston caliper. The same as a u13 bluebird turbo or w10/w11 avenir. These rotors are rare and cost lots.
After this you may as well got 5 stud.
I know because i've done all of those to my car