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jb000-
Googling it seems like some sort of magic do all product
It's a sealant that also bonds, and it sets hard but can still shrink / grow with heat / cold without cracking. It's water proof, shapable and you can paint it. It will fill large gaps and set and be quite durable. But you can cut it away with a knife once set. Roofers seem to like it for obvious reasons :)
Bit of a bodge solution, the right answer is probably flashing or even cutting a recess for the end of the gutter to sit in the wall, but that's proper pro level shit.
Howard
Can you unclip the guttering - either all of it or a section - and shunt it down so it closes the gap?
Then a small blob of some sealant maybe CT1 to seal it.