Service Pack SP171219 was recently released for GreenTools for PayPal (G4PP).  

This service pack allows the settable values of "refund_description" and "refund_reason" to be used before calling the #g4pppay_refundPayment() function.  These values will set the refund description and reason respectively.  

In testing we do see the description appearing in the refund details, but it doesn't appear the reason has been implemented yet.  Needless to say, it is available in the API no doubt they are planning on adding it somewhere.

As always, feel free to contact me with any questions.