Problem:

We want to override CalculateBaseMagnitude that without virtual:

UFUNCTION(BlueprintNativeEvent, BlueprintCallable, Category="Calculation")
float CalculateBaseMagnitude(const FGameplayEffectSpec& Spec) const;

Solution:
Before compilation, UnrealHeaderTool generates the virtual function for a BlueprintNativeEvent in the base class. The example from above would look like:

virtual float CalculateBaseMagnitude_Implementation(const FGameplayEffectSpec& Spec) const;

So in the derived C++ class, override (and implement) that function as usual in C++:

class PROJECT_API UGMMCCustom : public UGameplayModMagnitudeCalculation
{
    float CalculateBaseMagnitude_Implementation(const FGameplayEffectSpec& Spec) const override
    {
        float Ret = Super::CalculateBaseMagnitude_Implementation();
        
        // Add your custom implementation in the cpp file.
        
        return Ret; 
    }
}

Reference
https://stackoverflow.com/questions/55272245/how-to-override-a-blueprintnativeevent-function-in-c

幸运的人一生都被童年治愈，不幸的人一生都在治愈童年。----阿德勒