if you have working gals, you maybe able to do a little circuit testing with a la or probe, and build a device that would pass input values and record output values, if the chip is just being used as a simple address decoder, then it should be relatively easy to build a jedec file, check out wincupl (amtel ) , the output jedec should go fine to a lattice gal, most gal's can be re-written over ( as least the lattice gal16v8d i have)
the only time reverse engineering will be hard is if the incorporated latch/adders/sub-tractors into the logic.. ie, access a 0x08 input gives you say, 0x03 out, then you goto 0x09 and it gives you 0x04, but then if you go back to 0x08 it gives another value then what you had before, random noise input will repeatable output indicates that there shouldn't being any crazy logic.
also, if the schematic is available, it will make it easier to figure out the inputs and outputs, and then with the mame source, sort out address selects based on the input pins.
not all programmers will write them, i picked up a top853 for $50 on the bay that works fine burning gal's
is there any numbers stamped on the gal? maybe doing a google search may find a dump of one.