Git Extensions is a Windows GUI wrapper for Git. It reads the Git executable path and configuration from HKCU\Software\GitExtensions at startup.
The actual Git configuration (user name, email, aliases) is still stored in .gitconfig files — Git Extensions reads and writes those files, not additional registry keys.
Git Extensions integrates with Visual Studio and Windows Explorer (shell extension); the shell extension registers under HKCR\*\shellex\ContextMenuHandlers\GitExtensions.
The SSH client can be configured to use OpenSSH (bundled with Git) or PuTTY's plink.exe; the path is stored in the gitssh registry value.
RecentRepositories subkeys are numbered and contain the path of each recently opened repository.