G - stands for Git (or TFS, SVN, even CVS, which are all source / version control systems).
Git is a system for tracking changes to source code - sas, html, css, js, xml, etc. It is also a workflow for change management and integration testing, supported by vendor platforms such as Github, Gitlab, Bitbucket and more.
Typical git commands include:
git clone "some repo" - create a new local copy of a remote git repository
git branch BRANCHNAME - create a new branch (eg for building a new feature in isolation)
git checkout BRANCHNAME - switch to a different branch
git pull - get the latest version of the current branch from the remote repository
git status - scan ALL files in the repository to see which have been changed, and which are staged
git diff SOMEFILE - compare the changes of a particular file to the last commit
git add SOMEFILE - stage a file for commit
git commit -m "SOME MESSAGE" - commit staged files with a commit message.
When written properly, commit messages can be used to auto-generate release documentation and semantic version numbers - the article below is well worth a read.