This is such bollocks. The President does not have the authority to tell private agencies what policies they can set in regards to adoption. He does not have the power to dictate to state agencies. I do not know of federal agencies that handle adoptions.
Misinformation like this is meant to stir up radical protests.
He could remove federal policy that force private agencies to go against their beliefs when placing children for adoption. That is not a rollback of gay rights but a restoration of the rights of children and the proper place of the federal government in such decisions.