well you just go down load the source and then run the slackware script that builds the package. or you do like all people do in linux with out a package manager. Then Install it with package tools or in a terminal as root type installpkg the program
Down load the source read the install file and the ./configure make make install.
during configure it will tell you what your missing. install that then try again.
to build it from slackware source using Slackware scripts
you need all three files. when slackbuild is ran if there is a dependency problem it will give you the out put but if you do a full slack install you will not have to worry about 95 percent of all libraries are in Slackware.
good luck
If you have slackpkg installed you would open a terminal and type slackpkg install gimp