I worry about that question. I think about you, and about how you've obviously played the original Half-Life because you're not a MURDERER, and how you've obviously played Half-Life 2 because you're alive. But then I worry that you may have somehow missed out on Opposing Force - a Half-Life expandalone that appeared in 1999, the very first game from Randy Pitchford's Gearbox Software!
Starring the wonderfully dully named Corporal Adrian Shephard, it's a really splendid thing. It takes place concurrently with the original Half-Life, having you play as one of those shouty-man marines soldiers, occasionally intersecting with the first game's tale. Which is SO FLIPPING ACE! All sequels should be this! It also somehow featured a whole other alien race alongside the Xen, called Race X. Quite how Gordon didn't bump into these fellows isn't explained.
But here's the thing - it's a really damned good shooter. What a daunting task facing Gearbox, trying to follow-up world-changer Half-Life, and yet they completely hold their own. It doesn't match the original, obviously. In fact, it's at its best when it steps as far from HL as it can. By the second half, you're wielding some spectacularly fun alien weapons, and in some novel situations. At 10 hours long, it's a full game in its own right, and a lovely forgotten entry in the Half-Life universe.