Smart Launcher 6 actually is fully optimized to work with gesture navigation, even if your device says "Not supported by your default home app, Smart Launcher".
The problem is some firmware based on the first versions of Android 10 prevent gesture navigation to work with any custom launchers. In December 2019, Google released a patch for Pixel 2, 3, and 4. Samsung, One Plus, and other manufacturers distributed this patch to their devices with a software update starting from the start of 2020.
If you have problem setting navigation gestures, please check you are using the latest software available for your device. If yes, we suggest continuing using the three-button navigation until a new software update will be available from your device.
Today, some manufacturers like Xiaomi still haven't addressed the issue. You can check the relevant page for details.
Although Smart Launcher 6 actually is fully optimized to work with gesture navigation, some devices still do not perfectly support gesture navigation for third-party launchers like Smart Launcher.
If this is your case, there are only two viable solutions. You can stay with gesture navigation and wait for a future firmware update to fix your problem, or you can switch to three-button navigation.
Unfortunately, there is nothing we can do to fix the problem on our side. We are aware of the problem and we are as frustrated as you for the inconveniences it causes.
However, manufacturers are constantly working to fix this kind of problem so we hope the situation will improve soon.