If you don't already have a JEDEC file (http://www.jedec.org) containing the fusemap to be programmed into your GAL chip, many tools are available to create such a file.
A nice program for Windows 95 that allows to create a JEDEC file for all GALs that GALBlast will support. Download the free Starter version from Lattice http://www.latticesemi.com. Consult its documentation on how to create a JEDEC file.
A large program http://wwwzpt.tele.pw.edu.pl/Files/synar50a (66MB) that I have not evaluated.
Formerly available at Lattice, this one also handeled all GALs that GALBlast will support. A copy is contained on the CD of the book "Schaltungstechnik mit GALs", Bitterle/Nosswitz, Franzis' Verlag http://www.franzis.de ISBN 3-7723-4683-9
Available at http://www.atmel.com, WinCUPL can be used to produce JEDEC files for most GALs. But remember that GALBlast can not program ATFs from Atmel.
An older program running under plain old DOS that allows to create a JEDEC file for all GALs that GALBlast will support. Download the free version from http://zpthp.tele.pw.edu.pl/Files/easyabel/ and consult its documentation on how to create a JEDEC file.
A smaller program from 1993 available at ftp://ftp.armory.com/pub/user/rstevew/GAL/ that assembles equations for PAL chips and contains PAL2GAL.EXE to convert the JEDEC files to the matching GALs GAL16V8, GAL20V8, GAL20RA10, GAL22V10, GAL6001.
A program from AMD that will create a JEDEC file for all PAL chips. The JEDEC file for the PAL can be converted to a JEDEC file for a GAL by PALTOGAL.EXE. Download for free from http://www.engr.uky.edu/~melham01/ee481/software.htm
http://www.rit.edu/~ghzite/courses/0618-438/palasm/
http://www.ee.bgu.ac.il/~elinst/palasm/
Converts a JEDEC file for PAL chips to a JEDEC file for GAL chips. The original program from 1992 was available at Lattice, but has recently been deleted from their WebSite. So I have included a copy of PALTOGAL.EXE here.
http://www.vantis.com/ ftp://ftp.vantis.com/ (now Lattice)
http://www.er.ele.tue.nl/VvAcht/Personal/Personal.html
http://www.repairfaq.org/filipg/LINK/F_pal_gal.html
ftp://ftp.cs.indiana.edu/pub/goo/PLD
http://stud1.tuwien.ac.at/~e9225140/info.html
http://www.sh-elektronik.de/service/demo/galant.pdf (GAL test board)
ftp://ftp.armory.com/pub/user/rstevew/GAL/
http://www2s.biglobe.ne.jp/~elm/reports/pgal/report_e.html
http://www.optimagic.com/lowcost.shtml
http://ptolemy.eecs.berkeley.edu/
http://www.hszk.bme.hu/~fa218/Download/Lattice_download_cable.zip (ISP download cable)