You may do this by setting the GPIF transaction count register with the number of bytes desired. The transaction count register is a 32-bit register, a 32-bit value, GPIFTCB3, GPIFTCB2, GPIFTCB1, and GPIFTCB0. Therefore the GPIF can perform a maximum of 4 Gigabytes (1 to 4,294,967,295; 0 = 4,294,967,296) per transfer. These registers are set with the desired number of bytes and represent a live update of GPIF transactions.
When a FIFO read or write waveform is triggered on that FIFO, and the number of bytes is set in the transaction count register, the GPIF will transfer the specified number of bytes automatically. Please see the ez-usb technical manual sections 10.4.3 for more details