Stallion Free said:What happens to Black Mesa is canon in Opposing Force. Valve wanted to tie up that loose end for good. Race X and everything else is Gearbox's non-canon.
Do you have a source? Everything I've come across over the years has mentioned Gearbox's games not being part of the Half-Life canon, although it's not like I've researched the matter to any degree.
I also remember a GAFer saying that Gearbox and Valve had a bit of a falling out because the latter refused to acknowledge the former's games as canon, but "grain of salt" and all that.
Edit: Some Google'ing revealed that you're correct.