Op, you need to know that In most states, whether gay adoption is legal is made on a case-by-case basis by a judge. There are only 16 states that definitely allow joint gay adoptions (when a same-sex couple jointly petition for adoption).
I don't know any gay couples, but according to this article
If both birthparents are willing to terminate their parental rights the first step you need to take is to have a home study done. This determines if your home is a safe emotional and physical space for a child. The home study includes a criminal background check. It is done by a social worker, and is a required part of any adoption in the United States. The next step is the termination of the birthparents rights, and then there is required post-placement supervision of the adoption by a social worker. The last step is when you go to court to finalize the adoption
Read more:
How do I adopt my niece and nephew? | Adoption Questions
So gay couples go through the same procedure heterosexual couples go through in order to legally adopt a child.
Also, many adopt their nephews and nieces.