The grlevelx forum at http://www.grlevelx.com/owners/viewtopic.php?f=2&t=199&p=11300&hilit=gps+com4+com5#p11300 shows that only COM1 through COM9 are supported. Something about older versions of Windows performing very poorly with COM port numbers over 9.
I did see something in the forum about GR2AE supporting up to COM32 in a recent release but it does not appear that this feature was put into GRLevel3.
I did see something in the forum about GR2AE supporting up to COM32 in a recent release but it does not appear that this feature was put into GRLevel3.