First of all, GLTF 1.0 is not compatible with GLTF 2.0, so don't try to load GLTF 1.0 models with PasGLTF, but really only GLTF 2.0 models, like with the other GLTF loaders from the C/C++ ecosystem also now as well, GLTF 1.0 is simply obsolete now, since GLTF 2.0 exists. At least my (now fixed) PasGLTFViewer FPC build has no problems to load GLTF 2.0 models, so I guess, that you tried to load a GLTF 1.0 model and not a GLTF 2.0 one.

PasGLTF is already completely GLTF 2.0 base-spec-core feature-complete, minus non-spec-core extensions such as the Draco compression extension, in other words, the current further work is only tidying up, optimizing and refining work.

And the UnitGLTFOpenGL.pas is just only a code template, that means, you yourself must adapt this code template unit to your own needs (for example for multiple animations at the same time) and to your own engine, because a really-universal-requirements loader would be either too overkill or almost impossible, because each engine and application purpose has its own data structures and design concepts, so that the UnitGLTFOpenGL.pas is really just only a code template and not a unit for real use. Only the acutal PasGLTF.pas GLTF 2.0 loader&writer unit is intended for productive use.