Wooooooohooooooo! JRiver Media Center 13 running G-Force 3.9 on Vista 64 bit no less!!!! Yeah!!!!! Thanks guys and I'll be upgrading my account now when it comes up for renewal.
No, as far as I know and have read, it is not possible to get GF 3.8.3 to be recognized by JRMC 14. J. River made some changes to the way viz plug-ins are loaded starting with JRMC 13 (and additionally made more changes along the same lines to JRMC 15), and so we released GF 3.9 soon after JRMC 13 was released to address these changes. However, the current version of GF (3.9.4) is compatible with JRMC 13 and 14. The current GF beta (4.0b3) addresses compatibility with JRMC 15.