In the US, people with MD (psychiatrists) can diagnose, but so can people with PhDs or MS, or MSW (master level clinicians. You do not have to have a PhD or and MD to diagnose. Though in every situation I have heard, a MD's diagnosis will out weigh a PhD or MS/MSW's diagnosis. So what they say goes.

And as for transgender, you must feel that you are in a body that does not align with how you feel internally to be transgender. And sexual orientation (who you are sexually attracted to) has nothing to do with gender identity (whether you feel male or female).