Greg's father is 6'3", while his mother was 5'7" Both of his parents were health physicists at nuclear power plants. He was born in Ohio and grew up in Virginia, finishing at Massaponax High School. In 2007, his family moved to Cebu, the hometown of his mother. He quickly learned Cebuano and was convinced by his maternal uncles to play competitive basketball.
He studied and played for the University of the Visayas from 2007-2009 and took up Political Science. He was a dominant force due to his 6'7" height. He transferred to Ateneo de Manila University in 2010 and became an eligible player in 2011. In his two seasons for the Blue Eagles, he averaged 13.12 PPG, 9.82 RPG, 1.82 APG 2.85 BPG and 49.7 FG%, and he placed second in rebounds and blocks. He successfully teamed up with Kiefer Ravena and Nico Salva to lead Ateneo to two UAAP championships and completing a five-peat.
He declared for the 2013 PBA Draft. In the prospect camp, he was measured to have height of 6 foot and 11 5/8 inches and a wingspan of 85 inches. he had a vertical reach of 11 feet 6 inches and performed 40 bench presses, the most of all draft applicants. He also did 50 situps, 62 pullups and 18 pullups. After the workout, multiple PBA managers and analysts listed him as the no.1 prospect. Barangay Ginebra San Miguel picked him with the #1 pick in the draft.
Greg Slaughter was paired with Japeth Aguilar to form one of the most formidable duos in the PBA and was dubbed the "Twin Towers". In his first game in the PBA, he tallied 10 points (5 of 9 shooting) and 13 rebounds in 36 minutes of play.