Complex sentence: The only person until then who had been executed during the post- "Furman" period was Gary Gilmore, who volunteered to be executed in Utah, in 1977, effectively ending the national moratorium on the death penalty which had been in effect since 1967.
Split into simpler sentences: The only person to have been executed before then during the post- "Furman" period was Gary Gilmore, held in Utah, who volunteered to be executed in 1977. His death effectively ended the national moratorium on the death penalty executions that had been in effect since 1967.