How to sync the desktop to the app: import structure and also the transactions.
First go to desktop and do save as and save as an xml file ex: Business.xml
Copy that file to the phone, for samsung android, my file explorer works very good.
Now go to the phone app and do settings/accounts/import xml file.
Nothing will happen.
Next go to the app and select Open and browse to the place the xml file was saved, select it and open. It will then say importing xml.
All structure and also all transactions will appear.
And that is how you get your entire desktop file and with all transactions into the phone app.

It's very convenient to have all transactions on the app as well ... one can then refer back if needed and also see current balances for cash accounts as transactions are made.

And I presume the reverse also will work, export from the app as a file then import and open in desktop to update that as well.

I am using gnucash 2.6.7 on win XP and the latest app on Samsung S5 Android 6. Maybe Gnu cash can add this idea to their help file on the app. At any rate it worked for me, my app now has all structure and also all current transactions in it. Took 30 seconds to do.