With Android 9 on the Fairphone, it isn’t possible to hide the navigation bar just in the settings. what you could do, is download an app like “fluid navigation gesture”, “navigation gesture” or similar. In the most of this apps you find a step-for-step instruction.
If not, enable usb debugging (tap multiple times on the buildnummer in the software information and you’ll find the developer options where you can do this)
Download adb on your computer: SDK Platform Tools 版本说明 | Android 开发者 | Android Developers
open the path in your command (cmd) prompt [like: c:\User\Downloads\platform-tools] or terminal [like: cd /Users/username/Downloads/platform-tools]
connect your phone with your computer, you got to activate usb-connection by mtp.
You can test the connection with the command adb devices. If the answer ist “adb command not found” try something like ./adb devices. Perhaps you got to accept the connection on your fp.
When all of this is done, you can give the command
./adb shell pm grant com.[The place where your apps data is saved] android.permission.WRITE_SECURE_SETTINGS
Like this, you give this app the rights to hide the navigation bar. Before you uninstall your gestureapp, pay attention to activate the navigation bar 
.
A comment by Bartek Stokrocki to a video about exaxtly this proceder (Thanks to mobiscrub: youtube. com/watch?v=ZKEEJ0HE_bo&feature=youtu.be )
Just one thing to note here - saying that this is 100% safe is not totally true. By granting WRITE_SECURE_SETTINGS permission you allow app to modify system settings that are meant to be modifiable only by user and with his full awareness, like proxy settings or enabling untrusted sources to install apps. Long story short - if this permission would be granted to some malicious app, it could do a lot of harm, so be careful with it. Great video otherwise:)
Hope I could help you 