Gnucap is a generic platform for analog/mixed circuit analysis. But there is some extensibility beyond circuits.
Some uses are known to work with unmodified Gnucap.

- Discrete physics models are conservative systems and as such representable in verilog-a. Gnucap supports a subset of verilog-adms, which is conceptionally verilog-a.

- Ageing effects, such as material properties and breakdown model, simulation.

- Real time audio processing and/or wavetable generation.

- Neurobiology, "brain cells" in combination with body mechanics, simulation engine extensions.

Other uses may be less known. Generally with the domain specific plugins, Gnucap + Python (or Octave) is suitable as a replacement for most of the commercial "multidomain dynamical systems" modelling and analysis tools.