GBM connector can be forced via advancedsettings.xml.
First ssh in and use kmsprint to get the connected connector name of the screen.
Create advancedsettings.xml (replace "Virtual-1" with your connector name):
Code: /storage/.kodi/userdata/advancedsettings.xml
<advancedsettings version="1.0">
<!-- Debug Logging -->
<loglevel hide="false">1</loglevel>
<!-- No monitor selection on GBM but connector can be forced: -->
<videoscreen>
<monitor>Virtual-1</monitor>
</videoscreen>
</advancedsettings>
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Debug logging is enabled to ease the setup.
Perform systemctl restart kodi to test the configuration.