You can use this command in the directory where you downloaded the file to extract it:
tar xvzf RGBcommander_0403_beta1.tar.gz
"tar" is the program you need to extract .tar.gz or .tgz files in linux. "x" is the option to extract a file, "v" means "verbose" and will print the list of files it's extracting on screen, "z" will use gzip compression (the .gz in the file name), and "f" says that the next item on the command line is the file name to work with.