Gothic language
This language allows us to judge definitely about the East Germanic languages, for the other two of this subgroup lack texts, just having several inscriptions. Gothic states left us rich literature from their kngdoms existing in Europe in the 4th - 7th centuries AD. Gothic was spoken by the Ostrogoths of ancient Germany and Italy and by the Visigoths of eastern Europe and Spain. It was replaced by other Germanic and Romance tongues in the period between the 7th and 9th centuries AD. Except for a few Norse inscriptions in runes, records of Gothic are older than those for any other Germanic language.

Most knowledge of Gothic is derived from fragments of a translation of the Bible made by the 4th-century Gothic bishop Ulfilas (or Wulfila). The largest and most splendid of these fragments is the Codex Argenteus, containing about half of the Gospels. The Gothic alphabet, traditionally devised by Ulfilas, consisted of 27 characters: 25 modified Greek symbols and 2 runes.

Here are some main traits distinguishing Gothic from other Germanic tongues:

1) We know that Common Germanic had three voices: active, passive and medium, but only Gothic preserved all three, using mediopassive voice in the present and past tenses.
2) Gothic used the method of reduplication (double stem) in constructing the perfect stem of the verb. This feature is common with Latin and Greek, but is not seen anywhere else in Germanic.
3) Diphthongs were shorted into long vowels in Gothic, and ai > é, au > ó.
4) The strong verbs of the V class lost the suffix -j- (ligjan > ligan)

Gothic was extensively rich with preverbs, or verbal prefixes, which were quite productive in making new verbs. Gothic died leaving just a small dialectal group of Crimean Gothic which can hardly be considered as a descendant of this tongue.