Check this official NVIDIA Build out… You have to link you UE4 account with Github before, otherwise you can’t download it.
https://github.com/NvPhysX/UnrealEngine/branches
Here is the mini FAQ from NVIDIA, you can find this document also in the branch folder of the build
https://github.com/NvPhysX/UnrealEngine/blob/VXGI2-4.19/UE4_VXGI_Overview.pdf
I think it is already working try to build a simple scene, white wall, red ball… Sun shines on the red ball and white wall. Increase the indirect lighting intensity in the post process volume under VXGI Diffuse. Play around until you can see some color bleeding on the wall from the red ball. Check the Overview.pdf it should explain all you need to know.