As the title says. It is also not mentioned on the articles "Terrorism in Europe" despite it being one of the largest terrorist programs on European soil, and "Bologna Massacre" despite the fact that the materials for the bomb used almost certainly coming from Gladio.
I guess it really is Natopedia
Not exactly leftist, but WikiSpooks.com is a good starting resource on deep politics, which includes things like Gladio. Just be careful because it has attracted some antisemites since it's about conspiracies.