Related to recent comments.

The new update for Garry's Mod brought some changes to the Lua API, which seem to have broken the properties menu that should appear when right-clicking an object in the context menu (when holding down C).

A theory I have is that due to updates related to certain GUI functions, the current code that adds items to the properties tab is rendered obsolete due to those changes. I might be wrong, but this is my best guess.