I actually grilled my friend who loves Half-Life on this.

There's something like 10-20 years between 1 and 2 and it's implied Gordon is in stasis. Ignoring "if he just teleported into the future, after spending hours escaping Black Mesa, how is he able to continue functioning for another whole day?" question and assuming he just got a really long nap, we still have the issue of him not aging a day. Nobody comments on this, really. All his friends are getting older by years and he's aged exactly a day.

By Half-Life 3, all of Gordon's friends should be geriatric while he's still as young as ever.

Oh, and let's not forget that Gordon is described as a valuable asset. It's implied he's capable of doing a lot and making a difference, which is why he's being sent on missions. But Gordon would be dead were it not for his team. Kleiner and Eli Vance fill him in and give him gadgets. Alyx and Barney get him out of several binds. Gordon is only valuable because he has such great support. Without that, he'd be a dead man and worth nothing. So where's the supernatural protection from the G-man like Gordon gets? Nowhere. Who saved Alyx from the explosion at the end of HL2? Vortigaunts, not G-man. He only showed up to save Gordon before they got rid of him.

Half-Life can't go on after HL3. Why? Because all of Gordon's friends will be too old to go on saving the world and Gordon will still be the same age he was at Black Mesa.

The reason Half-Life takes so long between releases? So we can empathize with Gordon's friends who wait months or years between seeing him. The difference betwene HL1 and HL2 was years. The difference between Episode 1 and HL2 was only a few hours because it's a relative timespan. The episodes are closer together, or meant to be. Gabe is making us miss Gordon just as much as his friends do.