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`.
x_forwarded_for = headers.get('X-Forwarded-For')
        client_address = self.client_address[0]
        server_address = ':'.join(map(str, self.server.server_address))
        if x_forwarded_for:
            x_forwarded_for_list = (x_forwarded_for, client_address, server_address)
        else:
            x_forwarded_for_list = (client_address, server_address)
        return ', '.join(x_forwarded_for_list)
    def forward(self, method):
        data = self.data_bytes
        forward_headers = CaseInsensitiveDict(self.headers)
        
        if forward_headers.get('Connection', '').lower() != 'keep-alive':
            self.close_connection = 1
        path = self.path
        if '://' in path:
            path = '/' + path.split('://', 1)[1].split('/', 1)[1]
        forward_url = self.proxy.forward_url
        for listener in self._listeners():
            if listener:
                forward_url = listener.get_forward_url(method, path, data, forward_headers) or forward_url
        proxy_url = '%s%s' % (forward_url, path)
        target_url = self.path
        if '://' not in target_url:
            target_url = '%s%s' % (forward_url, target_url)
        
        if not forward_headers.get('Host'):
            forward_headers['host'] = urlparse(target_url).netloc
        if 'localhost.atlassian.io' in forward_headers.get('Host'):