This is a little helper for Grand Total which compiles the required numbers for your German advance VAT return ("Umsatzsteuer-Voranmeldung").
Download and extract this repository and make sure its called Umsatzsteuer-Voranmeldung.grandtotal. Double click it to install it automatically in ~/Library/Application Support/com.mediaatelier.GrandTotal3/PlugIns/Umsatzsteuer-Voranmeldung.grandtotalplugin.
In Grand Total, chose File > Send to > Umsatzsteuer-Voranmeldung (German: Ablage > Senden an > Umsatzsteuer-Voranmeldung). Select your paid invoices for the requested time frame and click "Ok".
This will create a text file with the aggregated net sums ready for copy and pasting into your ELSTER form.