[GNC] How to change default book/file to open in GC 2.6 under Ubuntu Linux?

[rsbrux]

I am using GC 2.6.19 (the standard version) under Ubuntu Studio 18.4.

I have just created a new GC book/file, but when I close it and reopen 
GC, it opens the old book/file.

ISTR having to set up something in some config file to get GC to start 
with a particular book/file open, somthing I only did because I couldn't 
find a way to do this with a shortcut containing a command-line 
argument, as I would normally do under Windows.

I don't find any documentation or help describing how to specify a 
book/file to open on GC start.

How can I make GC open my ne book/file instead of the old one