My GRF is a trivial modification of the original engines GRF written by
the tt-forums user DigitalFox. AFAICT DigitalFox never specified a
license, but as (s)he doesn't seem to object to my modification I'm
working on the assumption that both that GRF and mine are public domain.
OTOH "Public Domain" doesn't seem to exist as a legitimate copyright
status in Canada, so if you want an explicit license I'd go with the
Creative Commons Attribution 2.5 Canada License, with the work
attributed to "cnww" and DigitalFox.

In any event feel free to distribute and/or modify it however you like.