“I was always surprised at the fact that Dean trusted Gordon so fast and revealed so much after just meeting him, but at the same time it is very touching that he is desperate enough with pent up emotion to open up to this total stranger.”
I think the “pent up emotion” is the key here. Dean was carrying all the weight of losing John, trying to figure out what John’s warning about Sam meant, realizing that the Yellow-Eyed demon might still be after them (and, by the way, they no longer had the Colt, so they were essentially defenseless), etc., and he couldn’t talk to Sam about it. Dean really didn’t have anyone to confide in, so I’m sure it was a welcome relief to meet Gordon. Of course, he couldn’t tell Gordon “everything”, but at least he knew Gordon would understand why he was struggling so much with John’s death.