Starting from Android 10 (Q), gestures stopped to work with third party launchers. This is something that does not depends on us but a precise choice of the Android development team. On December 2019, the Android team released an update that fixes the issues for Pixel 2, 3 and 4. If you still have problem setting navigation gestures, please check you are using the latest software available for your device. You can read more here:
Starting from Android P, Google introduced a gesture system as alternative to the navigation bar. Unfortunately, only build-in launchers have full control over the new navigation system. 3rd party launchers like Smart Launcher have to rely on a fallback mode fully handled by the Android system.