GS2 has options to use several different geometry descriptions, including

a simple s-alpha model

a relatively accurate 11-parameter local equilibrium model, equivalent to a recent paper by Miller (1998).

interfaces to full numerical equilibrium from EFIT format files (there also exist ways to read results from JSOLVER, and several other formats).

interfaces to VMEC stellarator numerical equilibria.

There are detailed notes of the GS2 geometry information in geometry/g_short.pdf and the folder geometry. Also of interest are notes on transforming from flux-tube to annulus geometry for visualizations.

[I think what is here in the geometry folder is a more up-to-date version of the Bill's draft geometry paper, unless that version gets updated further. (However, Bill's draft geometry paper includes some extra stuff including extra references that aren't in the local geometry notes, so these two versions diverged before we started updated the version here. So these two versions might need to be merged for completeness someday. PDF and PS copies of the Bill's draft geometry paper from 2001 are in geometry/g_short_old.]