graphql.language.Field as its name suggests is the AST representation of graphql query text. And yes it can be parsed into a string with some tricks

The parser does NOT start from a syntax element of field but its can be tricked

    static Field parseField(String sdlField) {
        String spec = """ query Foo {
        $sdlField
        }
        """
        def document = parseQuery(spec)
        def op = document.getDefinitionsOfType(OperationDefinition.class)[0]
        return op.getSelectionSet().getSelectionsOfType(Field.class)[0] as Field
    }

parseField("f(arg : 1) { inner sub selection }") for example

The thing is Field and graphql.language.SelectionSet is not really a representation of the "executable notion of a field and its subselection"

They are the AST syntax elements. For example read https://www.graphql-java.com/blog/deep-dive-merged-fields

{
   foo
   foo
}

This is valid AST and will include two graphql.language.Field objects but there is only one runtime field

You need some other representation of a "field and its sub selection". With the https://github.com/atlassian-labs/nadel (our Atlassian federated query engine) we use graphql.normalized.ExecutableNormalizedField and graphql.normalized.ExecutableNormalizedOperation - this is a more complete representation of the "runtime" when you take out the syntactical elements

You can mutate this structure and then "print" a sub document from this to form "sub operations"

If you are just wanting to know "the simple runtime field subselection" then graphql.schema.DataFetchingEnvironment#getSelectionSet and graphql.schema.DataFetchingFieldSelectionSet is what you want.

See https://www.graphql-java.com/blog/deep-dive-data-fetcher-results