ADMesh is a GPL'd program for processing StL files. StL is a triangulated surface representation similar to the GTS as used by Gerris (e.g. in GfsSurface), and StL can be converted to GTS with stl2gts. StL is much more widely available as an output format by solid-modelling programs, e.g. BRL-CAD and Blender, but many programs output malformed StL which troubles stl2gts. This is where ADMesh comes in.
As a simple example, ADMesh can be used to convert a binary StL file to ASCII StL format to make it easier to inspect with
$ admesh --write-ascii-file=ascii.stl binary.stl