I know that GST_PLUGIN_SYSTEM_PATH env variable is needed for multilib apps, but is it needed for regular 64 bit apps so that they’ll be able to find Flatpak specific paths like
/app/lib/gstreamer-1.0
or
/usr/lib/x86_64-linux-gnu/codecs-extra/lib/gstreamer-1.0?
The runtime already sets a default value. In most cases you will be covered by that, if not and you need to override that, use the runtime’s value plus the extra paths.
flatpak run --command=printenv org.freedesktop.Platform//25.08|grep GST_PLUGIN_SYSTEM_PATH
GST_PLUGIN_SYSTEM_PATH=/app/lib/gstreamer-1.0:/usr/lib/extensions/gstreamer-1.0:/usr/lib/x86_64-linux-gnu/codecs-extra/lib/gstreamer-1.0:/usr/lib/x86_64-linux-gnu/gstreamer-1.0