Are Blue Shift and Opposing Force "canon"?

Solution 1:

There is no official Half-Life canon. Valve have not and do not intend to define what is and what isn't canon. Series' writer Marc Laidlaw stated that they do not "get involved in issues of canonicity", that "canon itself is non-canon", and that there is "no official stance".

Source: http://half-life.wikia.com/wiki/Half-Life_Wiki:Canon

Solution 2:

I'd say they're both canon. I'm no expert, but I have played all 3 games and you see Gordon in both of the expansions. Blue Shift, you see him being dragged by the HECU soldiers on the way to the crusher, and in Opposing Force you see him jumping into the Xen portal. I'd consider them both canon. (Personal canon, while the creators have opted out of saying whether they are, I think the common consensus from the fanbase is that they're both canon.)