Actually, if you look in ANY decent book on The Celts you will find those same germanic tribes and many more smaller tribes identified as Celts. That's really pretty basic Celtic History.
Absolutely not. Celtic = Irish, Scots, Welsh, Cornish, Breton, etc.
Germanic = German, Danish, Gothic, Vandal, Frankish, Anglo-Saxon, etc.
Two distinct subfamilies of Indoeuropean and two distinct historical cultures are represented here.