$ gem list gherkin cucumber
*** LOCAL GEMS ***
gherkin (2.2.0, 2.0.2)

$ gem install cucumber
ERROR:  Error installing cucumber:
   cucumber requires gherkin (~> 2.1.4, runtime)

The following command didn't work too

$gem install gherkin cucumber

You have to install gherkin 2.1.4 or 2.1.5 first.

$gem install gherkin --version 2.1.5

See "Specifying Versions" of RubyGems here