To fix: To build you need to manually clone and generate the tar.gz expected by the spec file. Spec files uploaded to Fedora's repo auto download the source code. I would fix this later.

git clone https://github.com/coyim/gotk3adapter.git
mv gotk3adapter gotk3adapter-728d69a7102fc174d4d7ef16eaf527e2601f0c69
tar zcf gotk3adapter-728d69a7102fc174d4d7ef16eaf527e2601f0c69.tar.gz gotk3adapter-728d69a7102fc174d4d7ef16eaf527e2601f0c69/
rm -rf gotk3adapter-728d69a7102fc174d4d7ef16eaf527e2601f0c69

To build this package you would need to install the golang-github-gotk3-devel package.

Building the package:

fedpkg --release f31 local