Git, Tower, and Sourcetree

I'm trying out both Tower and Sourcetree GUIs on my Mac and I have two very basic questions:

• Do either of these GUIs install the Git executables or do they use the Git installed on my Mac?
• Where would Git be installed on my Mac? At the command prompt I've typed "which git", but it clearly isn't in the PATH so no luck with that. It's weird because I swear it used to be in the PATH.

• Found in the Tower FAQ:

As Tower comes with a fully functional Git binary, you don't have to install Git on your system to use Tower. If you also want to use Git on the command line, though, we recommend you use the OS X installer or MacPorts (if present on your system).

• As for Sourcetree, I believe it requires Git to be preinstalled on your machine.
• If you recently upgraded to OS X v10.7 (Lion), check this out: Git on Mac OS X v10.7 (Lion)