This answer record describes the differences in meaning of TXKERR and TXRUNDISP between their use in the User's Guide and their use in hardware for a 2-byte transmit interface.
The bytes TXKERR[1:0] and TXRUNDISP[1:0] refer to are reversed in the GTP RocketIO User's Guide when the a 2-byte interface is used. TXKERR[1] indicates an invalid K character code was specified on TXDATA[7:0], and TXKERR[0] indicates the same for TXDATA[15:8]. TXRUNDISP[1] shows the current running disparity for the byte in TXDATA[7:0] and TXRUNDISP[0] for TXDATA[15:8].