You killed the final boss and were transported to an "interstellar subway car" where the G-Man explained things to an extent. He then gave you the choice to work for him, or "not" work for him. Choosing to work for him got you "Gordon Freeman : subject under further evaluation" or some similar quote and then the credits. Choosing to not got an apology from the g-man and then a pic of Gordon with 100 of the aliens pouring down on him.