What are the low level programming waveforms for the Atmel ATF16V8B?
The programming algorithm for standard GALs, including the 16V8, is documented on this web page. It's too long to reproduce in its entirety here, but there are a couple of "quirks" that will make it difficult to perform using a Raspberry Pi:


The GAL is a 5V part. The Raspberry Pi uses 3.3V I/O.
To program the GAL, you have to supply 12V to the EDIT pin, and an even higher programming voltage (14 to 16V, depending on the part) on another pin.


My advice? Get a prebuilt programmer to handle it for you. I use a TL866 USB programmer, which can handle GALs as well as many other programmable parts (parallel EEPROMs, serial flash, AVRs and other microcontrollers...). It's a handy thing to have around.