Is it possible to send a request body when using the method GET?

It’s possible (the request is not blocked), but it isn’t recommended, since the gateway ignores the body of GET requests. This follows section 4.3 of RFC 2616 about the protocol HTTP 1.1, which states that:

It’s possible to send data to the server in a GET request through query strings (i.e., through URL parameters that help identify what is being fetched from the backend). Additional information about the resource being accessed or the context of the search may also be included in the request headers, if the backend is expecting a specific header. In any case, information necessary to adjust the server’s response should never be included in the body of a GET request, as it will be ignored.