(x-posted from a rejected post on /r/history and because I noticed this was busier than /r/askhistory )
Guy died in the 1800's with factual text written on his gravestone since he did die in the war, I'm unsure if the gravestone was made right after his death or even decades later.
Photo taken after 2000ish by the look of the quality photo! Any help would be much appreciated :)
The gravestone would, I think, be a primary source when considering gravestones, but we can't really say it's a primary source for the deceased unless we know what role he had in the stone's construction.
Do you have a specific gravestone in mind? Dates of birth and death are generally considered common public information and don't requite citations. But, as /u/TRB1783 said, other information would probably require additional corroboration or context. Depending on the information and how you're using it, though.