"Stop leak" type additives are only a short term, get-you-home fix, and will only manage that if the gasket failure is between a cylinder and a waterway. If the failure is between two cylinders it will do no good at all.
Many of these additives claim to be a permanent repair, but if they were no-one would ever need to replace a gasket!
I'm afraid the only real fix is remove and replace.
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