Getting a 25 year old game to run on modern hardware can require additional steps. The support pages on GOG for Rainbow Six contained some useful advice that helped me when recently installing the game on a new machine. https://support.gog.com/hc/en-us/articles/214062225-Tom-Clancy-s-Rainbow-Six-?product=gog
In my case it involved several steps before all graphics issues were resolved, however this may vary due to the different hardware listed.
(1) Run in compatability mode for Windows 98
(2) Install DirectX package again
(3) Install dgVooDoo 2 wrapper files
(4) Force machine to pick the right graphics card (it was the onboard graphics card that runs the game graphics without problems, not the main modern one)
In two missions the files of the GOG installation were corrupt, resulting in invisible characters. I was able to resolve that by overwrting the two maps with files from the original game disk, but another forum topic describes a workaround for people lacking access to the R6 disk.
Hope this helps and best regards,