The IDE’s new highlighting behaviour is a little misleading, isn’t it? After all, a script commonly contains a function of the same name, but it is not highlighted so. I try to not touch highlighting too much since doing so can break themes/plugins that expected original class/token names.
GMLive.js is different from GMLive.gml - it’s for testing little snippets of GML without creating a project.
GMLive.gml integration is mostly akin to using it in regular IDE, but you get little badges indicating that there’s “live” code in files.