flycheck の checker executable を PATH や exec-path を Emacs (GUI モード) を操作して設定するのがめんどいときは flycheck-define-checker で上書きできる。参考: flycheck で C/C++ のエラーチェック定義を追加する

(flycheck-define-checker ruby
  "A Ruby syntax checker using the standard (MRI) Ruby interpreter.

See URL `http://www.ruby-lang.org/'."
  :command ("chruby-exec" "ruby" "--" "ruby" "-w" "-c" source)
  :error-patterns
  ;; These patterns support output from JRuby, too, to deal with RVM or Rbenv
  ((error line-start
          "SyntaxError in " (file-name) ":" line ": " (message)
          line-end)
   (warning line-start
            (file-name) ":" line ":" (optional column ":")
            " warning: " (message) line-end)
   (error line-start (file-name) ":" line ": " (message) line-end))
  :modes 'ruby-mode)

(add-hook 'ruby-mode-hook 'flycheck-mode)

flycheck https://github.com/flycheck/flycheck
2.3.1 Syntax checker executables http://flycheck.github.io/Usage.html#Syntax-checker-executables
3.1 Definition of syntax checkers http://flycheck.github.io/Extending.html#Definition
flycheck-define-checker (ruby) https://github.com/flycheck/flycheck/blob/0.15/flycheck.el#L4103-L4117
chruby-exec (Run a command under a Ruby with chruby-exec:) https://github.com/postmodern/chruby/tree/v0.3.8#examples