The macOS Migration Assistant in Applications/Utilities will transfer all your data to another machine. Migration Assistant Manual
If you want to migrate the data manually, find the correct folder below and copy it to the same place on the new mac.
In case your hard disk or SSD should have a failure, you can restore the data folder using Time Machine or your backup tool. The application (GrandTotal, Timings etc.) needs to be closed first. Time Machine Manual
Starting with GrandTotal 6 the .grandtotal files are saved in user selected location.
|GrandTotal 6||User picked location|
|GrandTotal 3, 4 & 5||~/Library/Application Support/com.mediaatelier.GrandTotal3|
|GrandTotal 1 & 2||~/Library/Application Support/GrandTotal|