Gencest stories are a sub category of Gen stories. A mashup of Gen and Wincest, it describes stories where there is no sexual or romantic relationship between Dean and Sam, but the story will focus on their intense emotional bond.
Given that Sam and Dean have a very close canonical relationship, some writers will note on their fic that there is "Wincest if you squint" e.g. even if a sexual relationship isn't explicit in a story, a reader could easily read the story as if there was.
See also brOTP.