GRAM is Graphics Random Access Memory.
The controller stores every pixel in its on-chip RAM.
Most if not all TFT controllers have RAM and you can read the RAM (unless the external electronics prevents it)
I have never managed to use a "no CS" display to its full extent.
Your Due has only got one hardware SPI peripheral.
Which means you have to choose whether you use HW SPI for display or SD card. And bit-bang SPI in software for the less important device.
It is a mystery why they chose to add Backlight control but omit CS.
David.