The gem is called 'responders'

actionpack/lib/action_controller/metal/mime_responds.rb

@@ -8,7 +8,7 @@ module MimeResponds
     module ClassMethods
       def respond_to(*)
         raise NoMethodError, "The controller-level `respond_to' feature has " \
-          "been extracted to the `responder` gem. Add it to your Gemfile to " \
+          "been extracted to the `responders` gem. Add it to your Gemfile to " \
           "continue using this feature. Consult the Rails upgrade guide for " \
           "details."
       end
@@ -16,7 +16,7 @@ def respond_to(*)
 
     def respond_with(*)
       raise NoMethodError, "The `respond_with' feature has been extracted " \
-        "to the `responder` gem. Add it to your Gemfile to continue using " \
+        "to the `responders` gem. Add it to your Gemfile to continue using " \
         "this feature. Consult the Rails upgrade guide for details."
     end
 

actionpack/test/controller/mime/responder_test.rb → actionpack/test/controller/mime/responders_test.rb

@@ -9,7 +9,7 @@ def test_class_level_respond_to
       end
     end
 
-    assert_includes e.message, '`responder` gem'
+    assert_includes e.message, '`responders` gem'
   end
 
   def test_respond_with
@@ -25,6 +25,6 @@ def index
       get :index
     end
 
-    assert_includes e.message, '`responder` gem'
+    assert_includes e.message, '`responders` gem'
   end
 end