GPU is additional resource and computing power while CPU is busy with something else.
Important is not only to use GPU (as I assume it is possible to program to use BIMx GI on GPU) - but what algorithms of global illumination render will use. The latest technology on renders is Metropolis Light Transport as variation of Monte Carlo bi-directional photon path tracing. Sounds like very clever words.
But apart of jokes - MTL is "unbiased" physical based renderer (light simulator), when programs like CineRender, Artlantis or Twinmotion - mimicking photographic effect with different tricks, pre-calculations, calculations, final gathering, antialiasing etc.
All details about Monte Carlo: https://en.wikipedia.org/wiki/Monte_Carlo_method