Try to see if this tutorial could be useful to you: https://www.youtube.com/watch?v=prwc95KTsvY
For the specular map creation in the tutorial a fairly simple program called Gimp is used, which you can download and install completely free of charge, without any requirements: https://www.gimp.org/. You can find the dds plugin for it here: http://libregraphicsworld.org/blog/entry/normalmap-and-dds-plug-ins-updated-for-gimp-2.8
But the procedure of applying the texture to the model for GTA 5 is unknown to me, so I hope you have some experience with that.