Gordon Freeman disappeared at the end of Half Life 1 for several years. His exploits in Black Mesa were witnessed by very few people, most of whom likely died when the site was later nuked. Most of his fights were not even witnessed by anyone, and even when he did have company, it didn't last very long. Maybe some of the staff could even have held him responsible for the resonance cascade.
Anyway, how did he end up being regarded as a legendary hero by the time he shows up in Half Life 2 ?
Did the "G-Man" pull some strings? Otherwise I assume that the resistance just needed a legend for inspiration, and they happened to choose Gordon. If they listened in to the radio chatter of the marines (I doubt many survived even if they did manage to listen in), they would know how much of a fight he managed to put up. However, this is just my speculation...