Gatekeeping is when someone takes it upon themselves to decide who does or does not have access or rights to a community or identity.

You are gatekeeping by saying that I do not have the right to use retarded as a slur because it is linked to the mental health community and unless I am someone with mental health issues or someone with retardation from MS or Polio, then I cannot use that word.

To which I replied, Don't fucking gatekeep me with your bullshit

If I want to use retarded as a pejorative that's got nothing to do with you or anyone else. If I want to say something is gay in a negative tone or if something is sick in a positive tone, again, that has nothing to fucking do with you. I'm not being racist, I'm not being bigoted towards people with mental disabilities, so with all due respect, have a nice day!