You need to dig a little bit deeper. Plastics are different (http://www.the-warren.org/GCSERevision/resistantmaterials/plastics.html). First, you need to separate them to choose the right solvent to soften them. There are amazing industrial machines are able to separate the shredded plastic pieces by density or color.
Can you provide a little bit more information what gel form means and why do you need that?
I agree with Robert, as more info needed. for example, you can dissolve plastics such as PCL or PLA in the different solvent including water in case of PLA, and probably gives a viscous solution. what sort of gel are you after for what application?