Hi,
It is CPU processing. For putting GUI through GPU, there is no existing function and you would need to implement it.
Another approach is to call cairo functions through nvivafilter plugin. Please refer to Tx2-4g r32.3.1 nvivafilter performance - #16 by DaneLLL
This can put GUI on NVMM buffer directly instead of copying NVMM buffer to CPU buffer ad then copy back to NVMM buffer. Although cairo functions are CPU processing.