According to MongoDB documentation, installing Mongo should install also the embedded GUI: "Compass" https://www.mongodb.com/it-it/products/compass but it doesn't happen. Perhaps it is due to the patch "no-compass". I'm wondering the reason for this choice. Could it be included in the ebuild (perhaps optional with a "compass" use value?)
It's a separate product, it would make sense to package it separately. The mongodb source itself does not include it, it just downloads it from the net.
There is an ebuild for mongodb-compass here for those looking for it: https://github.com/funtoo/dev-kit/blob/1.4-release/dev-util/mongodb-compass/mongodb-compass-1.30.1.ebuild
Added to ::gentoo https://packages.gentoo.org/packages/dev-db/mongodb-compass-bin