Dear Ruiliang,
Like for steel towers, I would typically assume a damping ratio of 0.5-1.5% for drivetrains in the absence of more information. But the drivetrain damping is specified in the ElastoDyn module of FAST as a torsional damping constant (DTTorDmp) rather than a damping ratio. The drivetrain torsional stiffness and damping constants (DTTorSpr and DTTorDmp) have been discussed a few times on this forum e.g. see: Drivetrain 2.2Hz frequency and natural frequency 0.625Hz def.
Best regards,