All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Your grandmother's brother is unmarriageable for you, whether he is her full brother or half-brother, from her father's side or mother's side. He is a maternal uncle for your mother and you, so he is an unmarriageable.
Allaah Knows best.