A:  The uncertainty in parallax measurements for Gaia's DR1 was
            very roughly, about 0.2 mas.


      If we adopt the very rough rule that one should
            only use parallax values which are at least
            10 times larger than their uncertainties,

               maximum "good" parallax  ~  10 * 0.2 mas

                                        ~   2 mas

            So

               maximum "good" distance  ~  500 pc