The verb should give you a hint--in this case, for instance, had it been ihr, the sentences would have been "Ihr mögt den Apfel" (correct me if I'm wrong). I don't know if there might be cases where this does not apply, but so far the conjugation of the verb has helped me figure it out.
As far as I know, 'Gern' means 'surley', or 'definetely', so the person that told you that not putting 'Gern' in probably thought that without putting 'Gern' in, it sounds like you were knew to German, and that you need to make the sentence longer. Adding 'Gern' in probably changes the sentence: "I have an apple" to "I do have an appel". I hope I helped, good question, you deserve a: HOOT! Many thanks.<pre>
-The German DL Owl</pre>