Gastrin releasing peptide (GRP) is released by the post-ganglionic fibres of the vagus nerve which innervate the G cells of the stomach and stimulate them to release gastrin. The vagus nerve is tenth of twelve cranial nerves and the only nerve that starts in the brainstem (somewhere in the medulla oblongata) and extends way down past the head, all the way down to the abdomen. ... In humans, gastrin is a hormone that stimulates secretion of gastric acid by the stomach. ...

This is the reason for the observed fact that atropine does not block the vagal effect on gastrin release.