I understand you're looking for information about RTGI (Realtime Global Illumination), likely referring to the shader by Pascal "Marty McFly" Gilcher, version 0.51.
There is no legal free download of RTGI 0.51. Support the developer if you want the software — it's reasonably priced for what it does. rtgi 0.51 download free
However, I must clarify: for legitimate use. It is a paid product available on Patreon (Marty McFly's page) for supporters. Version 0.51 is an older release, but still under the same licensing. I understand you're looking for information about RTGI