This was extracted from the wikipedia site on Drepung Monastery:-
Old records show that there were two centres of power in Drepung: the so-called lower chamber (Zimkhang 'og ma) associated with the Dalai Lamas-to-be, and the upper chamber (Zimkhang gong ma) associated with the descendants of
Sonam Drakpa, an illustrious teacher who died in 1554. The estate of the Dalai Lamas at Drepung monastery, called Ganden Phodrang, had been constructed in 1518 by Gendun Gyatso Palzangpo (1476–1541), retrospectively named and counted as 2nd Dalai Lama.
Penchen Sönam Drakpa (1478-1554 CE) in 1535 succeeded Gendün Gyatso (1476–1541) on the Throne of Drepung, both of them being major figures in the history of the Geluk tradition. By the time Sönam Drakpa was appointed to the Throne of Drepung (Drepung Tri), he was already a famous Geluk master. He had already occupied the Throne of Ganden (Ganden Tri) and was considered the most prolific and important Geluk thinker of his time. His successor was none other than Sönam Gyatso (1543-1588 CE), the lama who would receive the official title of the Third Dalai Lama (Talé Lama Kutreng Sumpa).
Before his death in 1554, Sönam Drakpa established his own estate, the Upper Chamber (Zimkhang Gongma), which was named because of its location at the top of Drepung, just below the Ngakpa debating courtyard "Ngakpa Dratsang".
Tibetan Buddhist Resource Center attributes the following name variants to Penchen Sönam Drakpa: "bsod nams grags pa [ primaryName ], paN chen bsod nams grags pa [ title ], khri bsod nams grags pa [Primary Title], rtses thang paN chen bsod nams grags pa [ title ], gzims khang gong ma bsod nams grags pa [ title ], this last one referring to the Seat of the Upper Chamber established in 1554. According to TBRC, his successors referring to the estate of the Zimkhang Gongma were Sonam Yeshe Wangpo (1556-92), Sonam Gelek Palzang (1594-1615) and Tulku Dragpa Gyaltsen (1619-1656) - closely connected to the famous story of Dorje Shugden. (Some say that Drakpa Gyeltsen was Sönam Drakpa’s second reincarnation, but usually he is considered to be the 4th incarnation of Panchen Sonam Dragpa). It seems to be commonly accepted that Dragpa Gyaltsen was the fourth holder of the "gzims khang gong ma" incarnation line. According to Tibetan Buddhist Resource Center, Zimkhang Gongma Drakpa Gyeltsen (gzims khang gong ma grags pa rgyal mtshan) has been his "Primary Title". Since the search for his reincarnation has been banned, he has been the last one.