No, it's not the GPU ISA. Complex chips tend to embed management cores, for example to handle power management or various configuration operations across the chip. NVidia used a proprietary ISA (Falcon) for such cores, and moved to RISC V. But this is independent from the GPU ISA itself, which remains NVidia proprietary. So in the end, in their embedded SoC you can find (at least) 3 ISAs: ARM for CPU, NVidia proprietary for the GPU, and RISC V for NVidia management cores. And it's likely there are more, for example for audio DSP and other IPs embedded programmable units.
From what I researched, Falcon is used on nVidia GPUs for everything _but_ the graphics, for example to enforce a security model. But it's a start. Probably they will start using for other purposes as well, for example video encoders and decoders. I very much doubt though they will scutte their existing GPU architectures for RISC-V yet.
It's not the internal ISA of their GPU but of a CPU they use in their designs.