I don’t understand a bit about Gongsun Sheng’s role in Water Margin!

He originally practiced in the deep mountains and old forests, and his master was Luo Zhenren, a man with great supernatural powers. Gongsun Sheng had lofty ambitions and set his sights on thousands of miles. After he learned a skill, he was unwilling to bury his life in an ancient temple in the mountains. He wanted to make achievements in the world, so he teamed up with King Chao Tianwang and other seven people to outwit the birth plan. This man respects his master and is devoted to his mother. He has an old mother at home and no one to take care of him, and his master is full of mysteries to prevent him from getting involved in the secular world, so he has not gone up the mountain to join the gang. Later, Dai Zong outsmarted Gongsun Sheng. Gongsun Sheng originally descended from the Tiangang Star and went up to Liangshan to join the army to obey God's will. In the end, he saw through the world, retired after success, and became a monk in Jizhou.

Also, watching TV series is just entertainment. If you really want to understand the classics, read the book Water Margin thirty or forty times before you can understand it.