There are various levels of gzip compression, ranging from fast and inefficient to slow and optimal. I’d like to see the WPT grade for “Compress Text” reflect the compression efficiency.
As of now, having even the most inefficient compression levels will produce an “A” grade, which may mislead people into thinking that there’s no more juice to squeeze.
From what I can tell, there is no direct way to determine the level of compression used on a particular resource. You’d have to take the ungzip’d version and apply different levels of compression on it to find the matching content size.