Download the XKB configuration for bqn here, and place it whith the other layouts:
curl https://raw.githubusercontent.com/mlochbaum/BQN/master/editors/bqn
cp bqn /usr/share/X11/xkb/symbols/
Add an entry for BQN in evdev.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
<xkbConfigRegistry version="1.1">
<modelList>
...
</modelList>
<layoutList>
<!-- Add this layout entry -->
<layout>
<configItem>
<name>bqn</name>
<shortDescription>BQN</shortDescription>
<description>BQN</description>
</configItem>
<variantList/>
</layout>
...
</layoutList>
...
BQN should be an available layout in the GNOME Keyboard settings (restart the Desktop environment if it isn't).