gistfile1.rb

# models/robokassa.rb
module Spree
  class Gateway::Robokassa < Gateway
    preference :mrch_login, :string
    preference :password1, :password
    preference :password2, :password

    def provider_class
      ActiveMerchant::Billing::Integrations::Robokassa
    end

  end
end

# engine.rb
app.config.spree.payment_methods << Spree::Gateway::Robokassa