solution:
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from (tx/rx_clkout[0]*) -to DataInxDI/DataOutxDO[0]**
*tx_clkout of the GXB, which is connected to the other tx_coreclk
** the actual input/output ports of the high speed serial data of the top entity. Every port has to be assigned.
In my case:
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[0]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[1]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[2]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[3]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[4]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[5]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[6]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[7]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[8]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|tx_clkout[0]" -to SecGtTxSerDataxDO[9]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[0]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[1]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[2]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[3]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[4]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[5]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[6]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[7]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[8]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_sec|rx_clkout[0]" -to SecGtRxSerDataxDI[9]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[0]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[1]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[2]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[3]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[4]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[5]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[6]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[7]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[8]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|tx_clkout[0]" -to UsrGtTxSerDataxDO[9]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[0]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[1]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[2]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[3]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[4]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[5]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[6]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[7]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[8]
set_instance_assignment -name GXB_0PPM_CORE_CLOCK ON -from "GX10ch:GX10ch_usr|rx_clkout[0]" -to UsrGtRxSerDataxDI[9]