A .gltf is several files, and the geometry is in a separate binary file. OFF is a short, readable text file that geometry libraries and research code read directly. glTF is at home in 3D on the web and in real-time engines; OFF is the usual choice for simple polygon meshes in research code and shape datasets.
The model is converted as triangles: lines and points are dropped, positions are used as they are, and no axis conversion is applied, so a model that is Y-up in glTF and Z-up in STL, OBJ and PLY is written exactly as it was read. Textures are not carried over, a glTF that points at a separate .bin file cannot be read from a single dropped file, and Draco or meshopt compressed geometry is refused with a message asking for an uncompressed export. Models up to 3,000,000 triangles are accepted.