I’m the author of that plugin and I can confirm there was an issue.
It was in the GM\VirtualPages\TemplateLoader::init(); method.
Issue was caused by an untested switch from array_merge to wp_parse_args.
They are similar, but I would have to inverse order of arguments moving from one to the other. Shame on me.
I fixed that in the Gist (source) now it should work.