At their very hearts, GSCE, R3E, rF2, and, as I recall, pCARS are all the same. They're all derived from ISI's pMotor. Where they differ is what data is fed to the core, how the output is displayed/interacted with, and differing approaches to the tire model. The tire model and the output presentation are the two things that influence how we feel when we use them.
So gearboxes are gearboxes, engines are engines, drag is drag, friction is friction, and masses are, er, massive. Change the masses and the inertias change, etc. As long as those numbers are good, then the end result will be in the ballpark up until you get to the tires because tires are still just shy of black magic.
The presentation to us in the driver's seat, is dependent on update rates and strength of force feedback before clipping occurs. Slow update rates, low available FFB are problems and there are different approaches to dealing with them.
GSCE stuck with proven technology and Niels is a whiz with it. R3E also stuck with proven technology and have several experts; slightly different presentation than R3E that's suitable for their goals. Update rates for both are low and they're prepared to handle low FFB. rF2 and pCARS added new tire technology and neither are completely at home with low update rates and low FFB, though they can be adapted.