According to many sources Gotti didn't appoint John D'Amato boss. They weren't always under the Gambinos either and for part of the Gotti era they were considered rivals. Gotti had hopes of controlling the DeCalvalcante Family and making them the Gambino's B-Team. In order to do this he reached out to certain members, John D'Amato was one of these guys, he conspired with Gotti & Gravano to kill Gaetano Vastola, who was appointed acting boss after Riggi was convicted 1990. Gotti himself was indicted and convicted, so the plan never went through. Vastola was sentenced to an 8 year prison term for extortion. And John Riggi himself appointed D'Amato as acting boss from in prison. D'Amato's reign didn't last, as I think around the same time Gravano had just finished testifying and spoke about he and Gotti's scheme. And supposedly being recruited by the Gambinos is one of the reasons D'Amato was killed, along with the rumor of him being bisexual.