can be found in the present web directory. (The "wget" unix utility is a convenient way to download a whole directory tree from a web page.) (The dates below generally refer to the dates of the most recent *.f90 source code files.)
http://gk.umd.edu/Gryffin/, (also available here) is from March 20, 1999 (at least this is true today, may be updated later).
gfcvs0105 is from August, 1999, and includes, for example, a bug fix by Bill Dorland for the the nparmom=3 case (an option which is rarely used). (I think these changes might not have made it into the following versions.)
gryffine is from December 2000. This was the last version of the code Mike Beer was working with, including equilibrium-scale ExB shear (small scale zonal flows self-consistently driven by the turbulence are always included in all versions of gryffin).
gryffine0105 is based on gryffine, with some small changes I made in May 2001 to the makefile or to source code (just to work around compiler bugs or make more portable, I think it shouldn't make any differences in results). (When comparing multiple versions with "diff", you probably first want to do "make clean", and remove logical links which point to different versions for different computers or different compilers.)
gryffin_snyder is from September, 1999. This is a version I got from Phil Snyder near the end of his thesis research.
While these versions are very similar, and we tried to maintain a single version over the years, there are some variations among these three versions as different people were experimenting with various effects. If you're interested in the electromagnetic version, start with gryffin_snyder. If you're interested in just an electrostatic version (with a bounce-averaged trapped-electron model and equilibrium-scale ExB shear), start with gryffine. Some of these versions are missing the standard set of documentation and test cases. There is more documentation and test cases in some of the other versions, in particular see gfcvs0105.