Google Assistant was introduced at Google I/O last year, as a more personal voice assistant than Google Now. Since then, it has been made available on several platforms - Pixel phones, the Google Home, Android TV, and Allo. But Assistant is still unavailable on non-Pixel phones without a special build.prop tweak. Now the Open GApps team has made enabling Assistant a little easier.
Starting February 3, all GApps builds will have the option to enable Google Assistant. If you're using the Gapps 'Aroma' zip, Google Assistant shows up as an option during installation. If you prefer one of the non-GUI installers, you can add the 'GoogleAssistant' keyword to the gapps-config file.
Google Assistant is marked as an optional feature right now due to it not having the full feature set of Google Now. You can download all Open GApps packages here, but keep in mind that only builds starting tomorrow will include the option for Assistant.