That the neuter gender in Gaelic was somehow lost on a ferry in the Middle Ages is a classic example of “fake news”. In fact, the Vikings stole it and that is why Norwegian, today, has a neuter gender. D
Ah no, we had our own Celts here in Germany in very past times. I think they simply left it behind when they moved on to the islands. However, they took the Dualis with them as can be seen from the funny ways Gaelic nouns behave when occuring in numbers of two ... This has vanished almost completely from German. Pity!