<<As far as I understand, the Firebird 3 LINGER value is actually a property of the database (file). How can I check if this is set and to what? I thought GSTAT -h might show it, but it doesn't.>>

AFAIK, you cannot "view" this setting.... however you can set it and adjust it using the following commands...
https://www.firebirdsql.org/file/documentation/release_notes/html/en/3_0/rnfb30-ddl-enhance.html#rnfb30-ddl-enhance-linger

Paul