I decided to try the Ubuntu Gnome distribution. I was surprised to find GIMP was not available in the Package Manager. I tried a few other installation techniques, but no success.

Two questions: 1. Why is an important package like GIMP not available in package manager? Most distros install it automatically.
2. How can I get it installed?

The "Muon Software Center" isn't showing all packages.

Solution :
$ sudo apt-get update && sudo apt-get install synaptic gimp

"synaptic" is the classic `Package Manager´.
Has a 'Refresh' button to update the package lists.
( Same as 'apt-get update' does.)
And : All packages will be visible.
`Gimp' is available for all Ubuntu versions :
http://packages.ubuntu.com/search?ke...searchon=names

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#1.

Packages are provided by people. If people have time then they create a way for other users to use a package manager type install.

Sometimes distros don't care for eula in third party software so they don't use it.

Sometimes other dependencies may have issues so they won't include it.

#2

Can build your own. http://www.gimp.org/source/

Can make a install that is independent on your system lib's in some cases.

Use knudfl suggestion to add in source.

In some software one can go to hosted site for generic linux version or even specific version.

In any of the ways you bypass package manager you run a very real risk of keeping your system sane.

Ref. post #3. http://www.gimp.org/ : An open source GNU application.

Gimp used to be included with the Ubuntu live cd.
Was omitted a couple of years ago (for space reasons ?)

Gimp is still in the main repo http://packages.ubuntu.com/trusty/gimp
.. and can be installed with 'sudo apt-get install gimp'

Ubuntu, gimp http://packages.ubuntu.com/search?ke...searchon=names

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knudfl posted answer really I was just elaborating on questions.