Привет, в описании плагина сказано:
Options: html
Type: Array|String Required value.
An array which can contain an array of files relative to your gulpfile.js, and which can also contain URLs. Note that if you are to pass URLs here, then the task will take much longer to complete. If you want to pass some HTML directly into the task instead, you can specify it here as a string.
т.е можно написать вот так:
html: ['index.html', 'posts/**/*.html', 'http://example.com']
