If you’re using GitHub for Windows, git.exe may not be in your PATH, but you may find it in a location like: C:\Users\<username>\AppData\Local\GitHub\PortableGit_<numbersandletters>\bin\git.exe
That’s the situation for me, in Windows 7 + version 1.0 of GitHub for Windows.
In Windows 10 it appears to be in:
C:\Users\<username>\AppData\Local\GitHub\PortableGit_<numbersandletters>\cmd\git.exe
( \cmd versus \bin)
From GitHub Desktop 1.1
The UI is different and the Git path now is in:
C:\Users\<username>\AppData\Local\GitHubDesktop\app-<appversion>\resources\app\git\cmd\git.exe
PS: AppData is a hidden folder by default.