Share endpoint between multiple services in Serverless

After following the example in https://github.com/serverless/serverless/blob/master/docs/providers/aws/events/apigateway.md#share-api-gateway-and-api-resources I noticed my project was returning the error The specified provider "undefined" does not exist..

No xxxxxxxx used in this example.

To fix the problem above, the following has to be done:

1. Main serverless.yml. Used to create the Gateway that will be shared among multiple services:

service: myservice-gateway # This service will create a gateway resource to be used among other services

#####
# Provider is to be used as per usual.
#####
provider:
  name: aws
  runtime: nodejs8.10
  stage: dev
  region: ap-southeast-2

#####
# Here is the new part:
#####
resources:
  Resources:
    MyGatewayName: # Custom name, use whatever you want.
      Type: AWS::ApiGateway::RestApi # Keep this line as it is.
      Properties:
        Name: MyGatewayName # Same custom name used before.

  Outputs:
    apiGatewayRestApiId:
      Value:
        Ref: MyGatewayName # Same custom name used before.
      Export:
        Name: MyGatewayName-restApiId # Name of the output of your restApiId.

    apiGatewayRestApiRootResourceId:
      Value:
        Fn::GetAtt:
          - MyGatewayName # Same custom name used before.
          - RootResourceId # Keep this line as it is.
      Export:
        Name: MyGatewayName-rootResourceId # Name of the output of your rootResourceId

The MyGatewayName-restApiId and MyGatewayName-rootResourceId will be used to set the other services.

2. Service that will use the main gateway:

service: myservice-a # custom name of my service

#####
# That is the part that is different from the example in 
# https://github.com/serverless/serverless/blob/master/docs/providers/aws/events/apigateway.md#share-api-gateway-and-api-resources
#
# The provider name, runtime and region HAS TO BE SET.
#####
provider:
  name: aws
  runtime: nodejs8.10
  stage: dev
  region: ap-southeast-2 # The region has to be the same of the main service.
  apiGateway:
    restApiId:
      'Fn::ImportValue': MyGatewayName-restApiId # Name of your restApiId set on the main service
    restApiRootResourceId:
      'Fn::ImportValue': MyGatewayName-rootResourceId # Name of your rootResourceId set on the main service

#####
# Functions as per usual.
#####
functions:
  ...

Do the same as above for all services sharing the same gateway and you are done!