Complex sentence: In the summer of 1851, George McKee hired Gibbs to accompany him on a treaty excursion in northern Californis Gibbs traveled to Oregon City and Fort Vancouver in July, 1851, then presumably sailed to San Francisco to meet McKee.
Split into simpler sentences: In the summer of 1851, George McKee hired Gibbs to accompany him on a treaty excursion in northern California. Redick McKee and the Humboldt Bay Region, 1851-1852 pg 203 Gibbs traveled to Oregon City and Fort Vancouver in July, 1851, then presumably sailed to San Francisco to meet McKee.