imagemagick-identifyis just a wrapper for imagemagick identify. You will need to install imagemagick separately from the gem.
If you are using debian / ubuntu you can do sudo apt-get install imagemagick.
If you are on a Mac, you may find this helpful.
If you are on Windows, you can find the executables to install here .
It looks like there are binaries for other platforms available on the last link.
imagemagick , , PATH.