Frederico,
My understanding is that Griffon is a framework written in Groovy to produce Swing applications, so I don't see how you'd use that with JavaFX since they seem to cover a lot of the same ground.
On the other hand, Dierk König of Canoo Engineering presented a talk at JavaOne titled, "JavaFX Programming Language + Groovy = Beauty + Productivity" on using Groovy and JavaFX together. It was a good talk, and Sun has made the PDF available at
http://72.5.124.65/learning/javaoneonline/j1sessn.jsp?sessn=TS-3968&yr=2009&track=javase, though you probably have to join the Sun Developer Network (it's free) to get it.
I hope this helps,
Burk Hufnagel
SCJP, SCJD, SCEA 5 "Any sufficiently analyzed magic is indistinguishable from science!" Agatha Heterodyne (Girl Genius)