Colonel William P. Smith (1833-1895) was Chief Engineer on Lee's Staff at the Battle of Gettysburg, although at least one source shows Smith's subordinate, Captain Samuel R. Johnston (1833-1899) as Acting Chief Engineer.
2nd source confirms that General Robert E. Lee named Colonel William Proctor Smith, as his Chief Engineer on April 9, 1863 and served in that capacity for approximately 1 year.
Field Armies and Fortifications in the Civil War: The Eastern Campaigns, By Earl J. Hess, page 22
But this source list William Proctor Smith, as Chief Engineer so that’s my answer.
The Battle of Gettysburg: A Comprehensive Narrative, by Jesse Bowman Young, page 383.