Gson: Not a JSON Object

Gson#toJsonTree javadoc states

This method serializes the specified object into its equivalent representation as a tree of JsonElements.

That is, it basically does

String jsonRepresentation = gson.toJson(someString);
JsonElement object = gson.fromJson(jsonRepresentation, JsonElement.class);

A Java String is converted to a JSON String, ie. a JsonPrimitive, not a JsonObject. In other words, toJsonTree is interpreting the content of the String value you passed as a JSON string, not a JSON object.

You should use

JsonObject object = gson.fromJson(value, JsonObject.class);

directly, to convert your String to a JsonObject.

I have used the parse method as described in https://stackoverflow.com/a/15116323/2044733 before and it's worked.

The actual code would look like

JsonParser jsonParser = new JsonParser();
jsonParser.parse(json).getAsJsonObject();

From the docs it looks like you're running into the error described where your it thinks your toJsonTree object is not the correct type.

The above code is equivalent to

JsonElement jelem = gson.fromJson(json, JsonElement.class);

as mentioned in another answer here and on the linked thread.