He was born in Chicago to German immigrants and attended Wheaton College, Illinois, whence he graduated in 1952.
In 1969 Paul Gast assumed leadership of the geo-science management of the Manned Spacecraft Center in preparation for Apollo mission sample return from the Moon.
He was one of the science consultant group known unofficially as the "Four Horsemen," along with Jim Arnold, Bob Walker, and Gerry Wasserburg.
During his career he pioneered the study of rare earth elements in examining the crust, mantle, and interior of the planet.
He led the development of the use of rubidium-strontium and uranium-lead radiometric dating methods for rocks, particularly for samples returned from the Moon.