This post has good tips. https://www.wikihow.com/Clean-an-Iron
Go to a fabric store and buy a product called Iron Off (or see if it's available on Amazon). It will take just about anything off an iron (I got melted plastic on one - don't ask!). Just follow the package directions.
Press an old wet white Towel. And expect to need to bleach the towel when you are done.
Hi Patrick: It depends on where the gunk is. If inside, you can run some white vinegar through the steam setting a couple of times, then some plain water. If it's icky on the base, then, while the iron is cold and unplugged, clean with white vinegar and a magic eraser. You can clean it while it's hot by running it back and forth across a towel that has vinegar on it. If it has sticky stuff on the bottom, you can use a loose mix of baking soda and water. Check the sole plate holes to see if they are clogged. If they are, use a toothpick to get the mineral build up off and then run white vinegar through the iron. Always follow up by running water through the iron to get the vinegar out. Good luck
If the gunk is on the outside of the iron (the heating element) turn the iron on high (no steam) to heat up. Put an old throwaway towel on your ironing board then put about 1/2 cup table salt on the towel and run the hot iron over the salt until the gunk comes off. I remember seeing my granny clean her iron this way.