I have a co-worker with a very unusual surname, one that also appears in my family tree. I have discovered that our great-grandmothers (my maternal, his paternal) were sisters.
I’m assuming this means we are cousins of some kind, am I right in that assumption?
You and this coworker are third cousins.
That was quick. 
Thanks Diceman
For future reference, the “degree” of cousins is calculated by counting how many generations one must go up to find the sibling relationships. Your mother’s sister’s daughter is therefore your first cousin, your grandmother’s sister’s granddaughter your second cousin, great grandmother’s sister’s great-grandaughter your third cousin, and so-on.
Here’s a convenient chart.