Gabe's just your friendly neighborhood angel, stopping by to give advice and reveal the future history of the world. Oh, and he's extremely terrifying—fiery eyes, glows in the dark, scares everyone away from the riverbank. Nothing personal—he just can't help it.
We know that it doesn't explicitly say that Gabriel is the dude by the riverbank, dressed in linen with the fiery appearance, but that's the way the rabbis have classically interpreted this and we're going to follow their judgment. Since Gabriel shows up in the previous chapters, where he is mentioned by name, we see how this makes sense.
This is the early part of Gabriel's career. Later on, he's going to show up and tell Mary that Jesus is going to be born, as well as dictate the Qur'an to Muhammad (if you're a believer in either of those religions). But in this part of his Biblical career, Gabriel is already making a name for himself by revealing secrets, interpreting mysteries, predicting historical events, and battling with the gods or angels (or whatever) of other nations. He's a bit like the "Prince of Persia"; there's nothing this guy can't do—except maybe make a burrito so hot that he couldn't eat it.