This is a dumb comment! Hybrid never mentioned anything about a GTS - infact, Hybrid specifically referred to "Turbo" and "RB20DET".Back to the question. I've done this swap, but it was into a GTS4, so I had to get an engine from a Stagea to get the 4WD sump. That ups the ante for a start. Including the Wolf 3D, I guesstimate it cost me around $8k all up. That also included a flywheel and clutch assembly (my new engine was from an auto, but I intended to do the flywheel / clutch anyway). It would probably cost maybe $5k for just an engine swap, although I did all the labour myself.
You don't need to swap gearboxes - my gearbox is happily handling the 180 Awkw (probably 240 fly kw) being pushed through it.
If you get a RB25DET with VVT, then you will definitely need the loom from the new engine. Otherwise, simply swap all the RB20 sensors into the RB25, and plug in a RB25DET computer.