There doesn't appear to exist an open library of GDTF fixtures at this time. Also, because GDTFs are binary files, they're not very easy to collaborate on. This repository contains a list of unpacked GDTF files (they're just zip files), and a script for repacking them into GDTFs. This ought to allow the community to contribute to an open GDTF library and collaborate on fixtures that are not fully defined.
If you want to contribute, just open a pull request for the fixture you'd like to define. A good starting place might be to improve this readme :)
If you'd like to repack all the GDTFs, clone this repository
git clone https://github.com/heliostate/OpenGDTFLibrary
cd OpenGDTFLibrary
bundle
./pack_gdtfs.rb ./unpackedGDTFs ./packedGDTFs
This will require you to have ruby and bundler installed
Otherwise, releases are periodically compiled and zipped up and posted in the releases section