The Celtic people are Germanic/Gallic. They moved away from Germany to Britain, not vice versa.
The Germans, in any case, were tributary to the Celtic speakers along the lower Danube.
Gaelic was brought into the British Isles as recently as 700 BC by Celts who'd former lived in the Black Sea region (see Danube). They took over Spain. Over the next few hundred years they cleverly enslaved their neighbors (today's Basques) and took them to Hibernia. Today's Irish and the Basques are remarkably identical in mtDNA and other markers, although their languages are vastly different.