There's actually an in-universe explanation for this.
You even hear Dr. Breen contemplate the same thing over the intercom during the mission in Nova Prospekt. How can Gordon Freeman, an untrained junior scientist, compete against the might of the Combine?
As you'll recall, even the G-man saw fit to retain Gordon Freeman in stasis until he was deployed via teleportation in HL2. The question is, why?
Put simply, when the player dies and respawns - so too does Gordon, canonically, in-universe.
Gordon basically gets unlimited "do overs" in any conflict. If he has a one-in-a-trillion chance of success, he only needs to respawn one trillion times to overcome the odds.
Crazy as it might seem, the game uses this mechanic explicitly during one Game Over sequence.
In Half-Life 2: Episode 2 during the mission "Riding Shotgun" you accompany Alyx on an escort mission. If you abandon her, you receive the following game over message before being reset:
"THE FREEMAN MUST PROCEED WITH THE ALYX VANCE, ELSE OUR STRUGGLE IS DOOMED TO FAILURE THOSE WHO DO NOT LEARN FROM THEIR MISTAKES ARE DOOMED TO AN ENDLESS VORTAL LOOP".
The key part of the message is "doomed to an endless Vortal loop". This message is from the vortigaunts, an allied alien race with psychic/teleportation powers. When you abandon the mission, contrary to their wishes, they literally respawn you back in time before the game over. They even threaten you with "doomed to an endless Vortal loop". The "loop" being an endless "groundhog day" like event where you must keep replaying the same event over and over again. This establishes the in-universe physics of respawning explicitly.
However, Gordon Freeman is human, and is not allied with the Voritgaunts until later. Why can he perform the respawn?
It was his central role in the Resonance Cascade, the central event to the entire Half-Life universe, that imbued him with this ability and made him a focus of interest ever since.
As you might recall, in the events of Half Life 1, Gordon Freeman was the lone person in the reactor chamber during the accident that lead to the Resonance Cascade. During the event he was seeming./y teleported randomly to Xen, then to a group a Vortigaunts, and then back to the lab.
For the remainder of the HL1, Gordon is shadowed by the G-man who is assessing his progress. When the game ends, G-man returns to "hire" Gordon for his "employers". When G-man conveys this deal to Gordon, he warns him that if he refuses he will be offered, "a battle he has no chance of winning".
This line is important. The only battle that could overcome the eternally respawning man would be one where the chance of success is exactly 0%. Indeed, if Gordon does refuse, he is teleported to a cavern, in some alien world, surrounded by hostile aliens.
So it would seem that G-man, one of the orchestrators of the Resonance Cascade, set in motion the series of events that would give him something of immense value... an unkillable warrior, a soldier who could fight a whole army and win, a man who can take on the Combine.
