You should get the desired result with something like this:
├── config.json <- site configuration
├── contents
│ ├── index.html <- these will just be outputted as-is
│ ├── gpg.html
│ ├── project.html
│ ├── blog <– each article has its own directory
│ │ ├── index.json <- this is your blog index at /blog/index.html
│ │ ├── look-a-new-wintersmith-blog
│ │ │ └── index.md
│ │ └── monkeys-are-really-cool
│ │ └── index.md
│ ├── authors <- author metadata, check author.jade
│ │ └── the-wintersmith.json
│ ├── css
│ │ └── main.css
│ └── feed.json
├── plugins
│ └── paginator.coffee <- paginator plugin
├── templates
│ ├── archive.jade
│ ├── article.jade
│ ├── author.jade
│ ├── feed.jade
│ ├── index.jade
│ └── layout.jade
└── views
└── articles.coffee <- view that lists articles
index.json is just a renamed and moved archive.json to give a /blog/index.html URL instead. If you want the default Wintersmith index instead of an archive layout, edit the file to use the index.jade layout instead of archive.jade.
If you change your current HTML files to Markdown and put them in the same spot, then they'll be outputted as HTML as your blog posts would.
You might want to add some sort of navigation menu to your article layout, too.
Edit: To create a static page, create a Markdown file in contents similar to the following:
---
title: GPG
author: baker
date: 2014-03-23
template: article.jade
---
Content
If you named this file gpg.md, it should be accessible at http://localhost:8080/gpg.html. Because we used the article.jade template, it expects an author and a date field for completeness (it would work without, however it would still include "Written by" without an author), but you could make a custom template that doesn't use those fields.