The solution was very simple: Someone raised this as a bug on the GSE GitHub site. [BUG]Interface/AddOns/GSE/API/OneOffEvents.lua:42: attempt to index global 'GSE_C' (a nil value) · Issue #1223 · TimothyLuke/GSE-Advanced-Macro-Compiler · GitHub This then pinged my phone to tell me about it. I then fixed it and released an update. Fixes in GSE happen when someone posts on GitHub the detail of the issue. The template on GitHub asks for the information I need to identify the problem. Without that information I can go WoW something happened at your end. With the requested information I can solve problems.
Lesson from This: Posting about a GSE bug here is like complaining to Google about a Windows bug. Not a lot is going to happen till someone tells Microsoft. This is detailed very specifically GSE - Support and where to get help