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`.
CORS_ALLOWED_HEADERS = ['authorization', 'content-type', 'content-md5', 'cache-control',
    'x-amz-content-sha256', 'x-amz-date', 'x-amz-security-token', 'x-amz-user-agent',
    'x-amz-target', 'x-amz-acl', 'x-amz-version-id', 'x-localstack-target', 'x-amz-tagging']
if EXTRA_CORS_ALLOWED_HEADERS:
    CORS_ALLOWED_HEADERS += EXTRA_CORS_ALLOWED_HEADERS.split(',')
CORS_ALLOWED_METHODS = ('HEAD', 'GET', 'PUT', 'POST', 'DELETE', 'OPTIONS', 'PATCH')
CORS_EXPOSE_HEADERS = ('x-amz-version-id', )
if EXTRA_CORS_EXPOSE_HEADERS:
    CORS_EXPOSE_HEADERS += tuple(EXTRA_CORS_EXPOSE_HEADERS.split(','))
LOG = logging.getLogger(__name__)
class ThreadedHTTPServer(ThreadingMixIn, HTTPServer):
    """Handle each request in a separate thread."""
    daemon_threads = True
class ProxyListener(object):
    def forward_request(self, method, path, data, headers):
        """ This interceptor method is called by the proxy when receiving a new request
            (*before* forwarding the request to the backend service). It receives details
            of the incoming request, and returns either of the following results:
            * True if the request should be forwarded to the backend service as-is (default).
            * An integer (e.g., 200) status code to return directly to the client without
              calling the backend service.
            * An instance of requests.models.Response to return directly to the client without
              calling the backend service.
            * An instance of requests.models.Request which represents a new/modified request
              that will be forwarded to the backend service.
            * Any other value, in which case a 503 Bad Gateway is returned to the client
              without calling the backend service.
        """
        return True
    def return_response(self, method, path, data, headers, response, request_handler=None):