Is this a duplicate thread of a thread created before. The thread created before is mentioned in the following link.
If so, Please carry on the discussion in the previous thread.
In the firmware you shared in this thread Multiple Thread in GPIF II for DR_data , i found that the configuration for serial out is 0x00001F83, /* CY_U3P_PIB_GPIF_SERIAL_OUT_CONFIG */
which is socket zero ( as mentioned in GPIF_SEROUT_THREAD_NUMBER_MASK (bit 3:2) is configured for data sink (from USB to GPIF).
As you are using two sockets (thread) in you firmware, this field should be 0. So that the socket is chosen internally (as per the thread)
So the configuration should be 0x00001F81, /* CY_U3P_PIB_GPIF_SERIAL_OUT_CONFIG */
I have tested this at my end. Refer to the results
PS: I have changed the DMA buffer size and endpoint size to 1024 (for testing purpose)
it was the exact answer. It works now.