Personal data
Gerhard Christiaan Rücker
Born 1722   Died April 5, 1780  
Professor Rücker
Johannes Gerhardus Christianus Rücker

Professor of Law at Utrecht University (around 1760) and Rector of that same institution in 1763-64. He was also, at some point Professor at Groningen.1

Shortly after his arrival in Utrecht, Boswell went to Rücker's lecture on September 20, 1763, before settling on following Professor Trotz's lectures instead.

Prof.J.U.D. J.G.C. Rücker (text in Dutch)
Boswell in Holland 1763-64

Unless otherwise noted, the source for a dated quotation of Boswell's is generally the corresponding volume of the Yale Trade Editions of Boswell's journals.