We did make some changes to JP2 format support in v13.2 that appear to have broken the terrain file support. I have those fixed now I think, although I only have 32-bit JP2 terrain files to test with so the fix isn't tested yet for 16-bit JP2 files. I have placed a new build at http://www.globalmapper.com/global_mapper13.zip with the latest changes for you to try. Simply download that file and extract the contents into your existing v13.xx installation folder to give it a try. If you are using the 64-bit v13 version there is a new build at http://www.globalmapper.com/global_mapper13_64bit.zip .
Let me know if I can be of further assistance.
Global Mapper Guru