Hi CJ, once you get the contact to budge or even lift an edge, there is a product Bunnings sells in the cleaning section called Oomph. It is citrus based and it is excellent at neutralizing the stickiest of gums. I had a commercial application where I needed to clean a machine covered in years of built up spray adhesive. We tested thinners, turps, metho and a couple of recommended dangerous chemicals, don't remember the names, along with a number of products off the shelf and off the shelf Oomph was the easy winner.
Cheers,
KJ