I want to know how to get gunk off from bottom iron fry pans

plastic bread tabs are the bomb for getting baked on gunge from the bottom of all cook and bakeware. the sharp plastic edge just lifts it off.

If they're like 'rescue' pans. That no amount of soaking, scrubing and such will get off. Start over. Take them to a autobody shop and ask about a 'chemical sandblast'. Or a normal sandblast.
It'll bring it down 'new' surface and you'll have to season them again.

If it's on the exterior...just go to town with whatever you have, sand papers, chore boys, brillo pads. etc.

you could also use steel wool (not brillo) instead of the chain mail if you don't want to buy it.

I have started to use a chain mail scrubber on my cast iron pans and it is a game changer! Way less elbow greese than my old salt scrub method and I think the pans' seasoning has actually improved with this method. It's really hard to ruin a cast iron pan, just be sure to dry the pans in the oven after use and cleaning to prevent rust.

First, try wiping some oil on the pan bottom, then sprinkle salt on it. Scrub vigorously with a paper towel, adding more salt as needed. When done, rinse with water, wipe dry, rub on oil, place in a 300 degree oven for half and hour to season.