The Nyquist rate is half the baud rate of a given signal. The baud rate is different from bit rate that it is the "number of changes in a signal per second" while bit rate is "number of bits per second". With this we can say that GMSL2 3Gbps link has a baud rate of 3Gbps and GMSL2 6Gbps link has a baud rate of 6Gbps since the number of bits is equal to the number of changes in NRZ.
Therefore the Nyquist rate of GMSL2 3Gbps link is 1.5GHz (3Gbps/2) while the Nyquist rate of GMSL2 6Gbps link is 3GHz (6Gbps/2).