I dloaded the ImageMagick....
I tried to use it.. but it seems it's command line only..
That is true - it is command line mostly. There are some basic GUI's for it, but nothing too fancy at this time.
Is there a gui for the app (for the mogrify command)
Not at this time.
There are some GUI applications for Windows that will generate Contact Sheets. One that I have seen is called ImageWalker (http://www.imagewalker.com), but I know nothing about the quality.
Leonard