I was under the impression that Spatterlight was actually "Gargoyle for Macs"...
Spatterlight is a very fine Glk implementation, but it is not the same as Gargoyle. I don't believe they share very much code at the library level, though I am not very familiar with the Spatterlight source. They have a similar set of interpreters, though Gargoyle's are more current since Spatterlight is not maintained.
Gargoyle uses Freetype to display fonts where Spatterlight and Zoom use native OS X font rendering. As a consequence, its output looks identical across platforms, and is more faithful to the font's design. It substitutes certain ligatures and replaces quotes with smart quotes. It uses subpixel rendering on LCD monitors and adds a lot of whitespace to make reading easier.
Because it doesn't use native text capabilities, some common UI features (such as scrollbars) are either implemented differently or not at all. This makes it difficult to compare to other libraries, which typically target one and only one UI toolkit. Readability is enhanced but at some cost to usability. Whether or not you find the program worthwhile will depend on which area you emphasize.
If you haven't used it before, it's worth downloading it to take a look.