would anybody know how to create a ballistic gel effect?

Fracturable FEM should do it.

PBD's are also pretty good at doing jello-ish type sim's, though for a ballistic gel effect as shooting it with a bullet, getting that type of expansion inside it, you will absolutely need to fake that with a separate object. There's just no way you're going to be able to simulate that level of transfer of energy, you'd need thousands of substeps per frame and you'd be simming for 25 years…