How to return the correct JSON from the server-side java to DataTables?

There are multiple different ways this can be solved:

Modify JsonElement tree

You can first use Gson.toJsonTree(Object) to convert ymList to an in-memory JsonArray and then wrap it inside a JsonObject to which you add your dtInfo properties:

JsonObject dtInfoJsonObj = new JsonObject();
dtInfoJsonObj.addProperty​("draw", dtInfo[0]);
dtInfoJsonObj.addProperty​("recordsTotal", dtInfo[1]);
dtInfoJsonObj.addProperty​("recordsFiltered", dtInfo[2]);

Gson gson = new Gson();
JsonArray ymJsonArray = gson.toJsonTree(ymList).getAsJsonArray();
dtInfoJsonObj.add("data", ymJsonArray);

String json = gson.toJson(dtInfoJsonObj);

Wrap data in separate class

Alternatively you can create a separate class, e.g. DtInfo, which contains the properties and the data and then have Gson serialize that. This approach is likely more efficient since there is no intermediate step creating a JsonElement tree.

class DtInfo {
    // Gson will use the field names during serialization; you can also customize them
    // using `@SerializedName`
    // Additionally you can make the fields private if you want
    public final int draw;
    public final int recordsTotal;
    public final int recordsFiltered;
    public final List<YthMmbrSectDtls> data;

    public DtInfo(int draw, int recordsTotal, int recordsFiltered, List<YthMmbrSectDtls> data) {
        this.draw = draw;
        this.recordsTotal = recordsTotal;
        this.recordsFiltered = recordsFiltered;
        this.data = data;
    }
}

And then create the JSON like this:

DtInfo dtInfoObj = new DtInfo(dtInfo[0], dtInfo[1], dtInfo[2], ymList);
String json = new Gson().toJson(dtInfoObj);

Also note that you can probably increase performance by storing the Gson instance in a static final field. Gson is thread-safe (see class documentation) and this way it will cache the type adapters it uses internally for converting the objects to JSON.

Additionally Gson provides toJson overloads which accept an Appendable. You could therefore pass the response.getWriter() to these Gson methods which avoids creating the intermediate String representation of the JSON.