What generation of cousins are we if his great grandma and my grandma are sisters?

Okay lets go in code. You are person Z and your relation is person Y.

Sisters A and B have children who are first cousins to each other (one of these is Person Zs mum).

These first cousins have children who are second cousins to each other. One of these second cousins is Person Z. The other cousin has a child. This child is Person Y.

Person Y and Person Z are second cousins once removed (the once removed shows that there is new blood so to speak in the family).

If person Z (you) were to have children, they would be th

ird cousins with person Y.

Does that make sense?