The name g is short for gaiji, which is the Japanese term for a non-standardized character or glyph.
Example
<g ref="#ctlig">ct</g>
This example points to a <glyph> element with the identifier ctlig like the following:
<glyph xml:id="ctlig"><!-- here we describe the particular ct-ligature intended --></glyph>
Example
<g ref="#per-glyph">per</g>
The medieval brevigraph per could similarly be considered as an individual glyph, defined in a <glyph> element with the identifier per-glyph as follows: