Many render engines implement the microfacet model for rendering surfaces that are rough, metallic, and/or transparent, a concept first published in "A Reflectance Model for Computer Graphics" This model implies that surfaces comprise a large number of microfacets, each with a random slope. The slopes of all the microfacets on a surface can be categorized with a facet slope distribution function.
Some render engines like EEVEE and Cycles allow users to select such function (called a "light scattering distribution function" in Blender), and the default is GGX.
GGX is presented in the paper "Microfacet Models for Refraction through Rough Surfaces."
In this paper's introduction:
We also introduce a new microfacet distribution, which we call GGX...
...but they never explain why they call it GGX.
Perhaps it's mentioned somewhere else?
