Use PALtoGAL Conversion Utility Software. A software utility from Lattice called PALtoGAL can be used to convert a PAL JEDEC file to a GAL JEDEC file. This software is also capable of changing the User Electronic Signature (UES) without changing the functional fuse data. A copy of the PALtoGAL conversion software can be downloaded from the Lattice web site at http://www.latticesemi.com/dynamic/view_document.cfm?document_id=303. (You will have to login to download the file.)
The file name is PALTOGAL.EXE.
If you do not find your device in the list of supported device, then there is no direct conversion to a GAL device and you will have to retarget your design.