There are diverging opinions on if being attracted to transsexual women makes you gay or not. Basically, it all depends on your definition of "being gay". In my opinion, the two big answers possible to that question will be given to you by two types of persons, those who like transsexual women, and those who don't like.
You are gay because whatever they do, they're still men
This is the answer basically given by those who don't like transsexuals. Not that they are homophobic, but they would feel really not comfortable being with a man, sometimes even gay jokes are offending them. Those people will say that what matters is genetics. If you are born with XY chromosomes, then you are to be a man for the rest of your life, period. Even if you look and feel more feminine and sexy than a real girl, you are not a real girl. And therefore, men who are attracted to transsexual women are to be gay.
Or maybe you are not gay, because what you like is femininity
The second way of thinking is that men who like transsexual women would actually never be attracted to men. Trannies are usually very feminine and sexy, often more than real girls, because they are pushing it very far. That makes attraction to transsexual women a very straight attraction, in that way. In this case, it doesn't matter that what pair of chromosomes she has, what you can see walking in the street or drinking at a bar is, after all, a gorgeous and sexy WOMAN.
And who needs labels, anyways?
I personally see myself as a straight man. I have GIRLFRIENDS, gorgeous and sexy that most of the men envy me, and I never have attitudes or behaviours that could be described as gay.
Labels just matter if you give importance to them. I don't need a label to know what I like and makes me happy. Call me gay, call me bisexual or whatever, that will not change a thing. If you are to ask this question, then you are the one who's got a problem with that, not me!