The brother of your great grandparent is your great great uncle.
Your great grandmother's brother is your great great uncle.
Your great grandmother's brother's great grandson is your third cousin.
your great uncle your grandmother's brother is your parent's uncle.... and confusingly enough when your parent is related to someone of the generation older than them (their uncle) it becomes your (great uncle) great relative.
Your grandmother's brother is your great uncle.
Your grandmother's brother is your great uncle.
Your grandmother's brother is your children's great-granduncle (or great great uncle). They are his great-grandnieces and great-grandnephews.
Your grandmother's brother is your great uncle. Your great uncle's grandchildren are your second cousins.
Your grandmother's brother is your great uncle and you are his great niece or great nephew. Your child is his great great niece or great great nephew.
You and the granddaughter of your grandmother's brothers, and sisters, are first cousins.
Your grandmother's brother's wife is your great aunt.
If your great great grandmother and your husband's grandfather were brother and sister then you and your husband are second cousins, twice removed.
The grandson of your great grandmother's brother is second cousin to one of your parents, and your second cousin once removed.