The following example `serverless.yml` fails to deploy on localstack:

```yaml
service: "localtest"

provider:
  name: "aws"
  runtime: "nodejs10.x"

functions:
  hello:
    handler: "handler.hello"
    
resources:
  Resources:
    ApiGatewayRestApi:
      Type: "AWS::ApiGateway::RestApi"
      Properties:
        Name: "${self:provider.stage}-${self:service}"
    GatewayResponseDefault400:
      Type: "AWS::ApiGateway::GatewayResponse"
      Properties:
        RestApiId:
          Ref: "ApiGatewayRestApi"
        ResponseType: "DEFAULT_4XX"
        ResponseTemplates:
          application/json: '{"errors":[{"message":"Custom text."}]}'
    GatewayResponseUnauthorized:
      Type: "AWS::ApiGateway::GatewayResponse"
      Properties:
        RestApiId:
          Ref: "ApiGatewayRestApi"
        ResponseType: "UNAUTHORIZED"
        ResponseTemplates:
          application/json: '{"errors":[{"message":"Custom text!", "extra":"Some extra info"}]}'
    GatewayResponseDefault500:
      Type: "AWS::ApiGateway::GatewayResponse"
      Properties:
        RestApiId:
          Ref: "ApiGatewayRestApi"
        ResponseType: "DEFAULT_5XX"
        ResponseTemplates:
          application/json: '{"errors":[{"message":$context.error.messageString}]}'
    GatewayResponseIntegrationTimeout:
      Type: "AWS::ApiGateway::GatewayResponse"
      Properties:
        RestApiId:
          Ref: "ApiGatewayRestApi"
        ResponseType: "INTEGRATION_TIMEOUT"
        ResponseTemplates:
          application/json: '{"errors":[{"message":$context.error.messageString}]}'

plugins:
- "serverless-localstack"

custom:
  localstack:
    stages:
      - "local"
```

The error on the localstack end is `Exception: CloudFormation deployment for resource type ApiGateway::GatewayResponse not yet implemented`.
forward_headers['host'] = 'localhost'
        forward_headers['X-Forwarded-For'] = self.build_x_forwarded_for(forward_headers)
        try:
            response = None
            modified_request = None
            
            for listener in self._listeners():
                if not listener:
                    continue
                listener_result = listener.forward_request(method=method,
                    path=path, data=data, headers=forward_headers)
                if isinstance(listener_result, Response):
                    response = listener_result
                    break
                if isinstance(listener_result, dict):
                    response = Response()
                    response._content = json.dumps(listener_result)
                    response.status_code = 200
                    break
                elif isinstance(listener_result, Request):
                    modified_request = listener_result
                    data = modified_request.data
                    forward_headers = modified_request.headers
                    break
                elif listener_result is not True:
                    
                    code = listener_result if isinstance(listener_result, int) else 503
                    self.send_response(code)
                    self.send_header('Content-Length', '0')