Hector was the Prince of Troy, a great warrior from Homer's epic poem the Iliad.
The poet makes a reference to the great classical hero to drive home the point of just how big a challenge it is for Gawain to take on the Green Knight. The guide tells Gawain in no uncertain terms what he's in for if he chooses to give battle to this man whose body is bigger than the best four in King Arthur's house, Hector, or anyone else.
Though Hector is killed in battle by Achilles toward the end of the Iliad, he is still presented as a formidable warrior. To be included by the guide along with the best four of Arthur's knights is high praise indeed, and this reinforces just how difficult it will be for Gawain to slay the Green Knight in armed combat.
In fact, the odds are so heavily stacked against Gawain that the guide tells him that he will be killed should he engage in battle with the Green Knight. But Gawain, being the brave, noble warrior that he is, doesn't flinch and heads on into the strange forest, a scary place that he will soon feel belongs to the Devil.