I have used the 4 inch Flex Tape to close up some pin hole rust out leaks on the flat bottom of steel gutters.
Wire brush all the crud and loose stuff off.
Remove all dust.
Clean with rubbing alcohol, let dry.
Patch on top side of gutter bottom, lay it on very smoothly, burnish it down to make contact, extend several inches beyond the leaky area.
It sticks very well, even if the gutter holds water.
I have been tempted to prime the old steel with some of the Flex Seal spray, but so far have not.
Easier than cutting a piece of flashing or sheet metal, bedding it in roofing cement, and attaching with a few short screws or rivets.