Oscar,

There is NIFTI_INTENT_VECTOR.  While the format states that it is for three-dimensional vectors, I think it would be okay to use for vectors of any dimension.

I have included a section of the GIFTI specification on extending GIFTI.  You could use NIFTI_INTENT_NONE or create a new intent as specified in the second paragraph (below).  Either way, you may want to put something in the metadata, perhaps a URL or email address for a description of the file’s content.  

I was unable to find a description of FreeSurfer’s GCS format.

John Harwell

12.0 Extending GIFTI 

This document describes the initial version of the GIFTI Surface Data Format. The data array intents in this version of GIFTI are those categories of data commonly interchanged between the various neuroimaging software packages. Developers are encouraged to use the GIFTI file format for new data intent types that do not require the addition of new XML tags. It is not necessary for developers to get approval of the GIFTI committee to add a new type of data intent. However, the GIFTI mailing list should be used to announce the new data intent to assess interest in the new data intent becoming an official GIFTI data intent.

When naming a new data intent, a process similar to that used for OpenGL extensions (http://en.wikipedia.org/wiki/OpenGL#Extensions) should be followed. In the case of GIFTI, the name of the new data intent should begin with the name of the neuroimaging software package creating the new data intent. For example, if the developers of Caret wanted to store latitude/longitude data in a GIFTI data file, the data intent would initially be named “CARET_INTENT_LATITIUDE_LONGITUDE”. If the GIFTI committee supported the addition of this new data intent, its name would change to “NIFTI_INTENT_LATITUDE_LONGITUDE”.