Validation for GET request params

I am trying to validate a GET request for a missing param.

The target API consumes a POST request, the gateway accepts a GET request, there is a body transform which converts this into the POST body, and Method Transform to convert from GET to POST.

But I want to validate and throw an error if a param is missing, it it possible to chain Validate JSON plugin to the results of Body Transform plugin.

Also, is it possible to do a Split operation on a string like this “john,doe” -> [‘john’,‘doe’],
I don’t know the extend to which the template supports go, I am not a go expert myself, but just checking.

And is it possible to have some sort of allowed parameters while validating JSON, say only
[a,b,c] are allowed and throw error otherwise