$ git describe --tags --long
find .git/objects -type f | grep g3c31c73
The "g" isn't a part of the commit.
Try looking for a commit
git show 3c31c73.
The "g" prefix stands for "git" and is used to allow describing the version of a software depending on the SCM the software is managed with. This is useful in an environment where people may use different SCMs.