Tested in multiple drives and folders. If the folder has a long name with spaces, the program will launch and close immediately. This behavior does not happen with any other program from PortableApps.com, that I use (and I use a lot of them).
I think it's the same bug this guy was experiencing!
EDIT.: I did the following tests:
C:\GIMP - OK
F:\GIMP - OK
F:\Portable\GIMP\ - OK
F:\Portable\Portable_GIMP - OK
F:\Portable\Edição de Imagens\GIMP - Does not run
EDIT2.: I did a new test:
F:\Portable\Edicao de Imagens\Portable_GIMP - OK
So the problem is not the space in the name, it's the latin characters