The grandchild of your great aunt is your second cousin.
If your great grandmother was his great aunt you and he are second cousins, once removed.
If the great grandmother of one person was the great great aunt of another, the two are third cousins to each other.
When one person's great grandmother is the another person's great aunt, the relationship between the two people is that of second cousins, once removed.
If your grandmother was his great aunt then you share a great grandparent and are second cousins.
If the great grandmother of A is the aunt of B's grandfather, then A and B are third cousins.
For clarity. let's call your grandmother's aunt "the aunt". Sequence of relationship: (1) The aunt's child and your grandmother are 1st cousins (2) The aunt's grandchild and one of your parents are 2nd cousins (3) The aunt's great grandchild and you are 3rd cousins
Your grandmother's sister is your great aunt. Your great aunt's daughter is your first cousin, once removed.
Your grandmother's sister is your great aunt. Her son's child is her grandchild. You and your great aunt's grandchild are second cousins.
They would be first cousins once removed. Your great aunt's children are the first cousins of your parent.
Your grandmother's sister is your great aunt.
Your grandmother's sister is your great aunt, her grand children are your second cousins twice removed. Genetically speaking this is barely a relationship - but if you were raised together as cousins the familiar bond can be tight.
3rd Cousins