On Wikipedia it says GPUs are an electronic circuit and on that page it says there are 3 categories of circuits, digital/analog/mixed.
https://en.m.wikipedia.org/wiki/Graphics_processing_unit
https://en.m.wikipedia.org/wiki/Electronic_circuit
https://en.m.wikipedia.org/wiki/Graphics_processing_unit
https://en.m.wikipedia.org/wiki/Electronic_circuit