The brother of your grandmother on your father’s side is the maternal uncle of your father, and a man’s maternal uncle is regarded as a maternal uncle for all his descendants. Based on that, your father’s maternal uncle is a maternal uncle for you, so he is one of your mahrams and you do not have to observe hijab in front of him. Rather it is permissible for you to uncover in front of him that which is usually uncovered in front of mahrams. 

Shaykh Ibn ‘Uthaymeen said: 

Note that a person’s maternal or paternal aunt is a maternal or paternal aunt for him and for all those who are descended from him. So your father’s paternal aunt is a paternal aunt for you, and your father’s maternal aunt is a maternal aunt for you; your mother’s paternal aunt is a paternal aunt for you and your mother’s maternal aunt is a maternal aunt for you. Similarly the paternal aunts of your grandfathers and grandmothers are paternal aunts for you, and the maternal aunts of your grandfathers and grandmothers are maternal aunts for you. 

Fataawa Islamiyyah, 3/131 

Shaykh Ibn ‘Uthaymeen was also asked: 

Is it permissible for a woman to uncover in front of her mother’s paternal or maternal uncle, or her father’s paternal or maternal uncle – in other words are these persons counted as mahrams? 

He replied: Yes, if a woman’s mother or father has a paternal uncle who is her father’s full brother or half brother through either his father or his mother, or she has a maternal uncle, then he is one of her mahrams, because your father’s paternal uncle is a paternal uncle for you, and your father’s maternal uncle is a maternal uncle for you. Similarly your mother’s paternal uncle or maternal uncle are uncles for you through ties of blood. 

Al-Fataawa al-Jaami’ah li’l-Mar’ah al-Muslimah, 2/596 

And Allaah knows best.