I recently downloaded the gNMR demo version.  I am curious as to how the chemical shifts are predicted because the predictions gNMR gave me for some simple model compounds were not even close to actual chemical shift values. Does the demo version use a limited chemical shift data base for its prediction?  I went through the demo instructions, but found no way to improve the prediction (other than to build a whole new database on which to base future predictions).

The DEMO version comes without database or database support and just uses a set of very primitive rules for shift prediction. These are intended as no more than the "better than nothing" approach and should not be considered as serious shift prediction tools.

The full version has a simple database included. That has to be set up (details given in the manual), but after that has been done prediction improves considerably. Also, you can add your own data to the database.