Is your feature request related to a problem? Please describe.
Encoding the request payload as a GET is extremely obtuse and makes creating new clients difficult and non-intuitive for other developers.
Describe the feature you'd like
Instead decoding the request in the Lambda, re-factor the API gateway implementation to pass along the payload json so that the API supports either POST or PUT as the verb and a normal JSON payload.
Additional context
Encoding payload in GET instead of using url parameters, as well as using GET for something that modifies data goes against RESTUL conventions in general.