I have an API running in AWS-API Gateway that I am followiing the Restful standards, as much as possible.
Now I have an endpoint that should receive a JSON and just validate it. It is not a CRUD opperation - I have no interaction with database here.
My first option would be to use a POST verb and pass the JSON in the body. But this is not a "Create Resource" action. And it is not a GET action since I am not querying any database and also I is not a good practice to use paylod in GET verbs.
What could be the best fit for this scenario in order to be attached as much as possible to the Restful standards?
You're right that it isn't CRUD, and that's ok.
POST is actually supposed to be used for updates, and sometimes to create, while PUT is the verb used expressly for creation of new objects in an Object orientated restful design pattern.
Your use case is definitely NOT object oriented; it is functional. In a functional service oriented architecture, POST is an excellent choice for your "RESTlike" use case.
If CRUD and true RESTful design patterns don't meet your needs, don't use them.