Benefit of having a GPU for adding watermark using FFMPEG using x.264
GPU acceleration is beneficial to encoding; that is its main use. But it has no benefit with x264 which is entirely CPU-based.

There is a H/W overlay filter but the speed improvements are negligible, AFAIK, because of the memory ops transferring data to and from GPU memory. It's primary benefit is for when the entire pipeline is GPU-bound.