I haven't been following GIMP development very closely, but it appears that in the latest development version, 2.7.1, "it is not possible to start up in single-window mode yet" (2.7.1 release notes). I guess it's possible to enable it after starting though.
There is a tutorial on How-To Geek explaining how to install 2.7.1 from a PPA and enable single-window mode.
I haven't been following GIMP development very closely, but it appears that in the latest development version, 2.7.1, "it is not possible to start up in single-window mode yet" (2.7.1 release notes). I guess it's possible to enable it after starting though.
There is a tutorial on How-To Geek explaining how to install 2.7.1 from a PPA and enable single-window mode.
A summary of how to add the PPA and upgrade Gimp: