Well I am totally confused.
Can you post a link to the Thea author article?

Gassian mirrors are also called Variable Reflectivity Mirrors
The link here shows an equation and a "bellish" curve, for describing the reflectivity, which seems to be done by optical coatings, with minimal or zero
physical curvature of the mirror.
http://www.standaphotonics.com/variable-reflectivity-mirrors/
http://www.altechna.com/product_details.php?id=927
https://www.lasercomponents.com/us/product/gaussian-mirrors/

- Brian