In rare cases the gpg --keyserver can fail in the process of installing RVM on MacOS. Luckily, there is a command you can run to circumvent this problem. Try using this instead:

curl -sSL https://rvm.io/mpapis.asc | gpg --import -

You should see in the results something like this:

  gpg: Total amount processed: 1 
  gpg: imported: 1

Which should mean that everything has worked as expected. Now try following all the remaining commands, starting with this:

\curl -sSL https://get.rvm.io | bash -s stable