Religious studies

how did the geluk school of tibetan buddhism begin and why did it eventually become the dominant school of tibetan buddhism? use simple words and simple structure to anwser this question.all the evidence should be in the source i give you.(2links and 2 pdf)
“http://www.treasuryoflives.org/biographies/view/Second-Dalai-Lama-Gendun-Gyatso/11114”
“http://www.treasuryoflives.org/biographies/view/First-Dalai-Lama-Gendun-Drub/10687”