cannot load such file -- google/api_client

[engineer-plus-plus]

I get this error

     LoadError:
       cannot load such file -- google/api_client

when I run this code

    require "gmail"

    Gmail.client_id = "my id"
    Gmail.client_secret = "my secret"
    Gmail.refresh_token = "rtoke"



    m = Gmail::Message.new(
      from: "myemail@gmail.com",
      to: "other@gmail.com",
      subject: "this is the subject",
      body: "this is a text body")

    m.deliver