Hi, bagha, and welcome to DiscussCooking! Hopefully, you will return for more than just this problem!
You are looking for something "free" to use on this "gook"....well, here is my advice!
First, NEVER use oven cleaner on your non-stick ANYTHING! That is a sure-fire way of permanent destruction of your cookware and bakeware!
Here is something that my Nonna taught me over 30 years ago, works EVERY time, and calls for no muscle other than the lifting and transporting of your pans and/or bakeware!
Place your gooked up pans inside a plastic garbage bag. You can use your "regular" plastic trash bags or even those from your markets, but just make sure there are no holes in them. Place your cookware in the bag, then place a ramekin, large glass ashtray, or other small GLASS bowl, into the bag, on TOP of one of the bakeware items. Fill that GLASS bowl with PLAIN amonia, then tightly seal the open side of the bag, either by tying it closed or by twisting and then tying with one of those paper covered wire twisty thingies. Place the bag with the amonia and your gooked cookware into your oven or any other confined area (like in a box with a lid and place the box in your garage or basement, etc.) and let sit over night or for 12 hours. After that time, peek at your cookware and see how it looks. It SHOULD, by this 12 hour mark, be clean. If not, re-seal the bag and let the cookware sit for several more hours. Depending on how thick and baked/cooked into your pans this gook is, will determine how long this will take. The gookiest can take a day. When this is reached, simply wash your pans/cookware with plenty of hot soapy water and thoroughly rinse.
I have had this gook appear from "who knows where?", and have used this trick from Nonna ever since, and without fail! Hope you have just as much success!