The term Germanic was derived from German using the family suffix -ic.
An alternative name is
Validity of grouping
The validity of Germanic has been well established on phonological, morphological and lexical grounds at least since Grimm 1819, which was one of the earliest comprehensive historical-comparative accounts of a language family.
Validity of classification
The Indo-European affiliation of Germanic has been well established since Bopp 1818, which established the Indo-European family. It has never been in doubt.
Harbert, Wayne Eugene. 2007. The Germanic Languages. Cambridge Univ. Press.