Who is poor Gregory the seventh (Pope Gregory VII?)?
I assume that you mean to be asking about Pope Gregory VII in this question. I will answer accordingly. Pope Gregory VII was one of the most important popes in church history because of the reforms that he pushed for and because of his involvement in disputes between the church and the secular government. Gregory VII was pope from 1073 until his death in 1085.
One reason why Gregory VII is well-known is the fact that he tried to implement reforms in the Catholic Church. There are two main reforms for which Gregory is known. First, he felt that the morals of the clergy were very poor. In an attempt to fix this issue, he placed great emphasis on enforcing celibacy among priests. The other reform that he is famous for is his attempt to abolish the practice of simony. Simony was the process of selling church offices to people. For example, a person might pay a bishop to let him (or his son or nephew) be the priest of a given parish. This practice was bringing the church into great disrepute and Gregory VII wanted to end it so that people would trust and respect the church more. Gregory VII is famous in part because of these reforms.
However, Gregory VII is also famous for his interactions with Henry IV, the Holy Roman Emperor. In those times, there was a great deal of competition between the church and the secular rulers for power in their society. The church felt that it should have power because it represented God on Earth. The secular rulers felt that the church should stick to moral and religious issues and let the rulers take care of worldly matters. Gregory VII twice excommunicated Henry for failing to obey what Gregory told him. Henry was forced to pay homage to Gregory at Canossa, making it seem as he was acknowledging that the pope had the greater power. This did not end the church-state competition, but it was an important moment in that competition.
Gregory VII, then, was a pope from the Middle Ages who was famous for his church reforms and his involvement in the competition for power between church and state.