Since the max data rate for GMSL2 is 6Gbps, the max required bandwidth is 3GHz. However, to reduce ISI, the selected relay bandwidth should be at least 1.4x the max bandwidth desired (3GHz). Considering this, you should be selecting a bandwidth that is flat from ~0 to 4GHz.
This way you will reduce any dB attenuation that may occur at higher data rates.