Git Flow is an extension to Git that provides extra functionality and simple commands that force you into a structured and proper branching model. Git Flow is not required and in fact all commands ran by Git Flow can be done using standard Git commands. Git Flow just makes everything easier. Git Flow only needs to be installed on the developer's machine (or wherever development happens) and not on any production server.
Git Flow can be installed alongside TortoiseGit without issue, but there are some steps needed.