Just updated it again: https://gist.github.com/generatepress/f5e4f6d1795cf6bb4c6a
The WordPress SEO plugin allows you to set the structure for the title tag for custom post types. Some custom post types, bbPress included, don’t display the title tag very nicely.
GP doesn’t do anything to the title tag, we leave it 100% up to WordPress and plugins incorporation custom post types. bbPress just isn’t developed to include nice title tags.