From another forum:
“ It's not glue - it is a rubberized sealing compound that manintains the integrity of the hermetically seaed can. The sealer seams to be more obvious from a result of using our can opener rather than from the traditional top-punch opener. In fact, in most cases, it maintains the seal of the can even after our can opener has broken the seal of the steel. You can often see it pulling away from the can a you remove the lid.
It is not at all toxic, at least, they tell us that.”