Hello, unfortunately the CaptureTexture method is missing from the current GpuRenderer interface. This is a mistake as the method is present in GpuRendererAsync (see here).
So, as of 0.9.6 there is no way to do that unless you use the GpuRendererAsync interface something like:
gpu_async = gs.GpuRendererAsync(my_gpu)
CaptureTexture will exist in GpuRenderer with the next revision.