Was Gordon in a simulator of the real world?

Does this stuff actually happen or is it some kind of simulation in Gordon's mind?

I ask because at the start of Half Life 2 you are sort of not in the train, then appear in the train, and then at the end of that everything around you fades and you're in a dark room kind of place?

Then episode 1 starts and you're again fading into "reality" from a darkened place....