This is the dedicated desktop/browser UI for the gdata notes system.
It is distinct from the JSONHTL notes store itself and distinct from the static HTML converter. GSUB stands for gdata-server user interface & browser.
The notes store is read and edited through the notes CLI and JSONHTL documents. The HTML converter renders notes into static web pages for deployment to a standard server; GSUB is the dedicated interactive browser layer for browsing the notes system.