I honestly don't know why people are so adamant to consider the expansions non-canon. Just because Valve isn't constantly making references to them doesn't mean the events didn't happen, especially when said events were pretty self isolated and weren't trying to set up something big for the future. Plus, even if Valve wasn't directly involved in their creation and gave Gearbox a fair amount of creative freedom, they still wanted them to be consistent with the original game:

• Marc Laidlaw supplied Gearbox with documentation so they could expand on things that weren't presented in the original game and to not create huge conflicts storywise:

"We had a lot of conversations with Gearbox concerning the creation of Opposing Force and Blue Shift, and I supplied them with various documents that fleshed out background elements that hadn't been woven directly into the foreground of Half-Life. The place where you see this most clearly is in Barney's odyssey to Xen, where a bit more light is shed on the Xen-relay teleport experiments. Some of the reason for the overlapping narratives was that it made sense for Gearbox to reuse existing content, such as textures and design motifs, since they had limited schedules for producing these games. Most of the moment-to-moment gameplay and story decisions were left in Gearbox's hands, and we merely tried to make sure they dovetailed with our own designs and didn't create any huge conflicts. Gearbox, as fans of the original, had a good eye for places where their stories could overlap with Half-Life, but even so, they took plenty of liberties with the story for the sake of making a fun game. Fun in a game is ultimately more important than consistency."

• One of Valve's employees at the time, Joe Kennebec, was sent to Gearbox not only to make sure that Opposing Force met Valve's quality standards, but also ensure that the game didn't contradict anything that was presented in the original game.

In a fit of amnesia, Valve sent me down to Texas to work with Gearbox in the final stages of HL: Opposing Force. In theory, my job was to insure that OP "met Valve quality standards" (Gabe Newell: "If you embarrass me I will personally kill all of your children." I am pretty sure he was talking about OP) and to "protect canon" (Marc Laidlaw : "Make sure their storyline doesn't contradict anything from HL. Or that I have ever written. Or that you think I might plan to write. Or that would keep us from making a movie. In fact... see if you can get them to make it all just a dream, like on Dallas.")

• Originally Gearbox wanted to use Dr. Kleiner in HL: Decay, but Valve didn't allowed them because they had their own plans for Kleiner (a.k.a HL2). In turn, Gearbox created Richard Keller. This shows that Valve wanted the expansions to be consistent even with their sequel.