There are also plenty of studies showing the opposite, as well as studies showing trans people being, on average, somewhere in between the male and female averages (on the specific area of the brain observed - generally different in each study!). The fact is, we just don't know.
And that's why GLAAD's definition is good. Trans people deserve respect and fair treatment regardless of what causes some people to be trans and others not to be trans. If we find out tomorrow that brain differences can't explain being trans, that doesn't change how trans people should be treated.
And it will probably be quite some time before we have a good answer there. We still don't know why some people are gay and some aren't, for example, and that has received far more research interest. The answer to that question, and to the trans question, will likely be quite complex, scientifically. But ethically, the answer is simple - just treat people nicely regardless.