The national legalization of gay marriage happened via the Supreme Court just last year. There is no reason at all it could not be reversed under a new court.

A lot more rests in Department of Justice policy. Look at what Justice has been doing to investigate and clean up racial bias in municipal police forces recently. They have consent decrees with Seattle, Cleveland, and others to try to change the way they do policing.

A new administration could easily abandon every one of those agreements, and the goal of removing racial bias from policing generally. It might even be likely--senior people in the Trump campaign have praised racial profiling as an effective and useful police tactic.

Edit to add: in general, social progress must be continuously defended. Nothing is every "done" in a democracy.

Yes, there is; it's "stare decisis". Even Dred Scott has not been reversed because of it.