I don't know too much about a GCU, but I can't imagine that there is much about them that couldn't be run with a good old fashioned analogue control circuit.
All analogue systems -- whether mechanical or electrical -- suffer from
aging (yes, that is a technical term). Simply put, analogue systems always
drift over time, continuously varying their performance until tolerance limits are exceeded.
When you combine an inherently analogue mechanical system with an analogue control system, you are essentially fighting a losing battle because both the system under control and the system doing the controlling will independently and jointly go out of tolerance. So analogue systems constantly requires external
tuning to keep their performance levels within an acceptable range.
Analogue systems are also susceptible to changing environmental conditions. The change in temperature from a hot ground tarmac to freezing flight levels are more than enough to affect the performance of analogue components (capacitors, resistors, amplifiers, etc.) So again, it's very difficult to maintain tight tolerances with analogue controls.
Digital systems, on the other hand, are not susceptible to these problems. If I set a certain digital memory parameter to the value of 128 decimal, it will not vary to become 129 or 127 over time. It will also remain exactly at 128 over its entire design temperature & environmental range. While memory can get corrupted -- and there are ways to automatically detect & correct this -- bits and bytes don't age, freeze or boil over.
The need to meet modern tolerance / precision requirements alone justifies the motivation to use digital vs. analogue techniques.
Plus are many other advantages to using a digital control systems. A single integrated digital controller can monitor, control and tune
hundreds of parameters simultaneously in real time -- something that's impossible (or at least impractical) to do with analogue controllers.
Analogue controllers also tend to be fairly "dumb". With digital controllers, manufacturers can implement sophisticated control algorithms to increase performance, economy, reliability, etc.