There are more sources for some languages than others. Most of the information about the Germanic languages we get from Roman references and translations. In most if not all of the languages there are translated Bible passages, due to the expansion of Christianity through Charlemagne and others.

For Gothic, most of the sources come from a bishop named Wulfila who translated the bible into Gothic, except for the Book of Kings. You can find his Codex Argenteus in a Swedish museum now. Other than Wulfila, we have other bible passages or commentaries in Gothic, as well as Gothic names found in Latin and Greek texts.

Most Old Norse text sources come from Iceland. There's the Prose Edda of Snorri Sturluson written about 1220, there's many poems about old Germanic gods and heroes, and there's the Islendingabok, a historical account of the Icelanders written about 1125.

The major Old Saxon document is the Helihand, a translation combining all the gospels into one book and changing many elements to reflect Saxon culture.

For Old English there's obviously Beowulf but also many other poems, and prose as well. We also have the Anglo-Saxon Chronicle, a collection of accounts of Old English history and culture. And of course there are Old English translations of Bible passages, particularly the gospels.

I can keep going if you want. I'm just summarizing from a book. Old English and Its Closest Relatives