I do not know of any car, any make that has a fuel tank drain plug. Some military and plant vehicles I believe have drain plugs.
I think you will have to pump the tank dry.
You don't expand/explain 'gunk'. It could be that you will need to remove the tank completely. Then you can rinse/flush with water and then fully drain down and dry out.
Another option, depending on the tank, then then will be a top access plate where the fuel level sender and outlet & return plates are located. You can remove this unit plate and have a good 6 inch access hole. If the tank is not baffled then a long thin arm will be able to wipe and clean the internal surfaces and finally mop up the residues at the bottom of the tank.
My W109 Mercedes has a fuel tank drain plug. I removed the tank from my Fiat 130 saloon to do some welding a while ago. I have a feeling it did indeed have a drain plug although I may be wrong, it's quite a while since I did it.