I had the exact same problem when setting up RVM on my new Mac this morning:
$ ruby -v dyld: Library not loaded: /usr/local/lib/libgmp.10.dylib Referenced from: /Users/psoshnin/.rvm/rubies/ruby-2.1.3/bin/ruby Reason: image not found
To solve the issue I first ran:
$ brew update && brew upgrade
Then I ran:
$ rvm reinstall 2.1.3 --disable-binary
And it installed successfully for me.
$ ruby -v ruby 2.1.3p242 (2014-09-19 revision 47630) [x86_64-darwin13.0]
Hope this helps.