Mass conversion of images to GIF

ImageMagick can do it:

convert -delay 100 -loop 0 image*.png animation.gif

Replace image*.png with whatever your images are named.

Install it with:

sudo apt-get install imagemagick

Beware that you'll need a lot of RAM for 600 files.

Converseen

...is a good GUI tool for converting and compressing images. It might be able to help you here. I'm aware of the ImageMagick answer, this is a GUI solution.

Its website is http://converseen.sourceforge.net.

Ubuntu 14.04

Available in the default repositories.

sudo apt-get install converseen

Ubuntu 13.10 and earlier

Not available by default but there is a (probably not dedicated) PPA:

sudo add-apt-repository ppa:ubuntuhandbook1/apps
sudo apt-get update && sudo apt-get install converseen