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.

wget https://github.com/gotk3/gotk3/archive/v0.4.0.zip
unzip v0.4.0.zip 
rm -rf v0.4.0.zip 
tar zcf gotk3-0.4.0.tar.gz gotk3-0.4.0/

To build this package you would need to install the glib2-devel, cairo-devel, gtk3-devel packages.

Building the package:

fedpkg --release f31 local