Sandstone guttering

Obtain new block of sandstone. Have at it with an angle grinder, to remove anything that does not look like guttering.

Cheapy way might be to get a roll of bitumen-backed flashing tape ( looks like lead flashing ) and stick a strip on the inside of your guttering. If the guttering is clean and dry, and not powdery, and the tape is applied properly on a warm day, it should last quite a while. Of course, it will tend to obstruct the flow of water and debris slightly, so as another poster suggested, the best repair is new stone. Another cheapy way might be fibreglass mat and resin.

Andy.