How to make Gimp run as a single window?

Ubuntu 14.04 and higher

Open GIMP, and go to Windows > Single-Window Mode

Ubuntu 12.04

Single-Window Mode is only available since GIMP 2.8+. You will have to install GIMP 2.8 from a ppa if you want Single-Window Mode. Just follow the next instructions:

1. Add the GIMP PPA. Open the terminal and copy/paste the following command: sudo apt-add-repository ppa:otto-kesselgulasch/gimp && sudo apt-get update
2. Update GIMP using Update Manager or apt-get.
3. Open GIMP, and go to Windows > Single-Window Mode

Now you have a single window! For example, my GIMP main menu:

Select "Single-Window Mode" from the Windows menu item. The setting will persist between GIMP launches, and is only available for GIMP version 2.8 and over. ( How do I get GIMP 2.8? )

There is an unofficial application to use Gimp 2.x in a single window mode called GIMPBox.

You can find all the info with installation guide about it here. It seems a bit complicated to install, but you will have a single window Gimp 2.6.

Personally, I would suggest you install Gimp 2.8. It's a stable release so there should be nothing to worry about. I use it too and the single window mode is awesome! Simply follow the instructions espectalll123 has given to install it and you are good to go.