Open GApps Can Now Enable Google Assistant in Your Custom ROM
Android: Most custom ROMs don’t come with Google apps preinstalled for legal reasons. Open GApps helps you install them in the new ROM and now it can enable Google Assistant for you as well.
Getting Google Assistant on a non-Pixel phone is relatively straightforward , but a little awkward. If you are using a custom ROM , you can now select an additional radio button that will modify your build.prop file for you. This isn’t the easiest way if you want to keep the stock ROM that was on your phone, but if you’re already using a custom ROM it’s a handy new feature.
Open GApps Adds option to enable Google Assistant during installation | XDA