First of all, I'm very happy to see that someone made this post. I was going to get round to writing this up as a mega-post along with some alternative routes they could have taken with Gek 'Lhar.
One thing that amused me is that I felt he had more character development than any of the Spartan IV's seen in Spartan Ops, his devotion to the Covenant was unquestionable. All I hope now is that we see Gek 'Lhar return in future Halo media, it'd be very interesting to learn of his past career with the Covenant Empire.
Theory wise, I'd like to think Gek is still alive and made it off Requiem before being destroyed. Perhaps they could slip this in with the shot maybe knocking him unconscious or something similar. I know he definitely wouldn't play dead, as that would be incredibly dishonourable, and from what we know that is the exact opposite of his traits.
I think 343 could easily pull this off, it's a sci-fi for goodness sake! Hey, it worked for Boba Fett, right? It just seems like an utter waste. Not only would it give us a rich and interesting character, but it would create a very credible rival for FireTeam Majestic, as I'm sure Gek would seek out for vengeance. This would be something far more interesting than what has been done with Majestic Team, and the Spartan IV's in general. Bringing him in as a recurring opponent for Majestic.
I've asked Catalog a a few questions about Gek a week or so ago, I'd like the construct to answer them as it seems Gek 'Lhar has actually stayed in the communities minds as quite an unforgettable character. Then again, I could see them saving these facts and Gek's history for us to uncover in future media.
All in all, I'd love to see Gek 'Lhar return into Halo media. Please make it happen 343 Industries.