I talked to my host today about GZip, and got some interesting info on the performance side of things.

I was told that GZip has its use for very large sites like Yahoo (I guess that's why Yahoo includes gzip as a performance tip), but for my site the benefit is minimal. For this reason my host doesn't even do it.

So what does this mean for my Zen Cart store, which has GZip turned on? Well, apparently GZip is still performed by Zen Cart's PHP code, just not by my host (just paraphrasing, I know little about these things interact with each other). However, CSS and HTML do not go through PHP, so they are not zipped. If I want to I can always upload compressed versions of these files (anyone tried this?).

Anyway I think I'll follow the experts advice and forget GZip.