Natasha,
I believe you did more than the previous owners, otherwise you would have found flyers, newspaper lost section or neighbors etc. that would have said "Hey that must be the Grey that just went missing X number of days ago".
At his point, I'm with the others, keep the Grey, He's yours. The owners in my opinion abandoned him or did not try very hard to find him.
Karma to you for being so vigilant at "Doing the right Thing" and now opening your home to a lost Grey in need of a loving person and family such as you. :-)