Feature Enhancement - aws-cloudformation/cloudformation-coverage-roadmap#308

Workaround:

Functionalities which are not natively supported by the cloudformation could be solved with the help of custom resource cloudformation where you could specify a lambda function ARN to invoke any logic and get back results to cloudformation state objects.

Steps:

1. Download the attached lambda-function python code and deploy it in same region as cloudformation stack. The lambda function takes VPN Resource ID as a input argument from cloudformation and gives back the TransitGateway attachment id as response.

2. Download the attached sample cloudformation and use it as a base or use similar approach to reference the TransitGateway Attachment ID from the custom resource. Please have a look at the comments in the cloudformation manifest for the use-case of each resources.

Notes:
A) Make sure the lambda execution role have enough permissions to invoke EC2 endpoint describe calls.
B) Any further enhancement on code logic customization could be done within the scope of 'def lambda_handler()'. In contrast, 'def send()' takes care of constructing cfnresponse to signal back the cloudformation stack with results whenever it gets invoked by cloudformation stack.

Things to Consider:
In the above steps, we have deployed the custom lambda function outside of the respective cloudformation stack, As I consider this function as a general purpose use by other future stacks. However please feel free to create the function on the same stack if you like to have the same.

References:

1. Cloudformation custom resources - https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/template-custom-resources.html
   
2. Boto core Describe call - https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/ec2.html#EC2.Client.describe_transit_gateway_attachments