It is not at all clear, even in the gospel of Mary, that this is the case.  It is one thing to say that the gospel shows that Jesus favored Mary, that he entrusted to her some teachings that were not given to the wider group of disciples, or that she understood Jesus’ teachings perhaps better than the other disciples.  But it is something else to say that he intended for the future of his Church to be in her hands.  Moreover, even if the gospel of Mary actually said what Brown alleges, this would not by itself mean that it is a historical fact.  It is quite possible (indeed likely, in my judgment) that Jesus did not designate anyone to be in charge of his Church.  Indeed it is quite possible (indeed likely, in my judgment) that Jesus said nothing about the establishment of a church in his name.  On this point even the canonical gospels say virtually nothing; the word “church” is used in only two verses in all four gospels (both in Matthew; 16:18 and 18:17).  The idea that Jesus appointed someone as the sole leader of a church to be established in his name is of so little importance in first-century Christianity that three of the four gospels written in that century say nothing at all about it, and the fourth has only one cryptic line on the topic.  This was an issue that later generations of Christians became obsessed with.