Does differential impedance between D+ and D- not only depends on separation between and wire gauge of D+ and D-, dielectric constant of the insulation, but also the GND wire and shielding?

I want to connect two EVMs which use PCIe connection by using the impedance controlled USB cable at hand.

However, I don't know if I should also connect the GND and shielding of the USB cable(of course, I will use common ground for two boards). If the differential impedance between D+ and D- does not depend on separation between D+, D- and GND, shielding, I think I can leave these two wires floating.