When using fast attack and especially release times, GR is hard to quantify. While it is true that one must not rely on the numeric values they are still useful for reproducibility. Is there a way to show peak GR within a time window, or to smooth such readings in general?
There are no options for what a-compressor can display.
There are many more sophisticated/featureful compressor plugins available. a-compressor exists because (1) it works fairly well (2) it covers a basic need that many users have, without them having to decide which other compressor to use.
If you have more specific needs, it would be worth seeking out a different plugin.