Transgendered individuals are considered to be the gender they feel they are (in trans-sensitive circles at least). So a biological male would be referred to (generally) with female pronouns, like her and she and is considered to be trans-female. If she was attracted to men, she would be labeled as straight, if she was attracted to women she would be labeled as a lesbian.
Based on my knowledge of transgender people, if a man changes their sex and is attracted to males, this person is a heterosexual trans person. Technically speaking, you shouldn't even consider this person to be gay in the first place.
Gender identity and sexual orientation are two different things. Gender identity would be what gender we see ourselves as. A man who changes their sex would mean that their gender identity is actually female. Being trans has nothing to do with what their sexual orientation is but rather that they feel they were born into the wrong body. If they happen to be attracted to males, this transgender person would be straight since they are female on the inside.
Don't worry if you still don't get it, it was hard for me to wrap my head around this at first too. If you want more information on this, feel free to read: www.quora.com/Can-someone-be-transgender-and-gay-at-the-same