There are two kinds, Field Programmable Gate Arrays (FPGAs) and Mask Programmable Gate Arrays (MPGAs). The inside of every gate array is just that - an X-Y array of logic gates of various sorts. You customize the gate array by interconnecting gates in an interconnection layer on top. In concept, this is just like using 7400 or CMOS logic, except you get to use a gate at a time. What could be better? Your very own custom logic chip!!
Gate arrays come in various sizes (that is, number of gates) and you pick your size to match your application, then use software compilers to compile your logic circuit into connections on the array, and from the array to the I/O pins. With an FPGA, you do this with a flash memory layer on top of the chip. The memory layer remembers which is connected to what. There are some EEPROM FPGAs that have to be programmed every power on by a uC as well. The huge advantage here is that you can reprogram them. But they are slower, and have to be reprogrammed.
MPGAs are the Caddilac. They are as fast as custom hard logic chips in most cases and need no power-on tending or programming. But that "mask programmable" thing means that the array is personalized by a metal mask ON THE TOP OF THE SILICON CHIP ITSELF. This can only be done in the semiconductor fab with an entire silicon wafer of several hundred chips at a time. Making a mask is very expensive; putting it on silicon is even more. MPGAs always come with (or came with, when I was in the biz) a Non Refundable Expense of $50K and up; "up" means literally whatever. I saw NRE bills of $250 - $500K on our MPGAs.
And it means NON REFUNDABLE. Your junior engineer made a mistake hooking up logic gates? Sorry. We put YOUR metal layer on YOUR wafer, and here are your chips, working or not. But we'd be happy to do it again for you if you want to try again.
MPGAs are perfect for many things if you can get over the necessary design, simulation, software verification, NRE, and hardware verification testing. But if you can't fund those things, don't even think about it. They're the classical "if you have to ask, you can't afford it" kind of thing.
I would guess that having done this once, Digitech was not eager to fund a new set of design, verification, NRE, and manufacture on a replacement chip when the maker of the gate array took it end of life. And that means they're telling you the truth - there aren't any. In fact, no one except Digitech or the original supplier of the chip ever had any of them. Another chip with the same type number that isn't personalized the same way is at best useless and at worst would further damage other stuff.
Repairing that whammy pedal is simple - find an buy another one, put the replacement's electronics inside the damaged one's housing. That's the only way to repair it.