It is difficult to say for sure without seeing all trace tracks in that frame, but looking at the screenshot I think it is waiting on Rendering.
I see Render Frame waited 8.6 ms (possible due to the PSO precache shown in the track bellow). And then spend the rest of the time doing rendering work.
The game thread track seems to wait until this rendering work is complete, as both the render and game thread track finish at the same time.
The advice @3dRaven gave is correct, you usually try to see how the tracks line up to see what work is being done.