The chip you have needs a 21-volt power supply during the programming process. This is common with very old UV EPROMs. My guess is the GAL G540 does not support this, as it is a USB-powered programmer (USB only provides 50 mA at 5V). Sorry about the confusion
this is a pin-compatible chip that is in current production. It retails for about $4 USD in single quantities and is available for immediate purchase at the link above. It's also available at most other distributors like DigiKey. See if your GAL programmer manual says it supports this guy. My guess is it does.
Another nice thing about this chip is that it's an EEPROM, which means Electronically Erasable PROM. If you ever need to change the patterns in your machine, you can pop this chip back out, erase it with your programmer and program it again with something new. There's no need for a UV lamp to erase it. Each chip is good for at least 100,000 erase / program cycles. And trust me, you'll snap the pins off of this thing long before you hit 100
![Smile :)](./images/smilies/icon_smile.gif)
Finally, don't be shy about asking about old tech around here. We're a very warm and welcoming community, and we love to tinker with old tech.
Best of luck!