We know quite a bit about Gothic. We have a significant portion of the bible that was translated into it, along with some other shorter texts and inscriptions.
Anglo-Saxon kingdoms in England were more recent in history, and they left more direct linguistic and cultural heirs that would be more likely to take an interest in and preserve old writings in the language.
By the mid 8th century, the last place ruled by people who would have considered themselves Goths was the narrow strip of northern Spain ruled by the kings of Asturias. Gothic had only ever been spoken by a small minority in Spain, and I'd if it survived into the 8th century, it wasn't destined to survive much longer than that.
You can actually read most of the Gothic corpus here: http://www.wulfila.be/gothic/browse/
We understand Gothic fairly well. It's very easy to read. What we don't understand well is native Gothic syntax, as most of the corpus is translated from Greek. The Gothic tends to mirror the underlying Greek syntax.
Gothic texts were destroyed by mainstream (trinitarian) Christians because it was the language of the heretical Arian Gothic Church. We don't really know to what extent Gothic was used after different Gothic-speaking groups entered the Roman Empire. Some scholars believe that the Visigoths were already speaking Latin primarily by the time they settled in Hispania, but that they were using Gothic as a liturgical language up until Reccared I converted to Catholicism. There are two land deeds in Gothic from the Ostrogothic Kingdom of Italy that suggest it was still in use there. We have a manuscript regarding the Gothic alphabet from fairly late, the 8th or 9th century, associated with Alcuin.
We also have a short quip preserved in the Codex Salmasianus that reads:
Inter eils Gothicum scapia matzia ia drincan / non audet quisquam dignos educere versus.
That is:
Amid the Gothic "Hail! Let's get [something to] eat and drink" nobody dares to put forth decent verses.
In "proper" Gothic:
"Hails, skapjam matjan jah drigkan!"
This text is part of anthology from the Vandalic Kingdom in North Africa. The author likely doesn't care about the distinction between Goths and Vandals, and is simply calling the language Gothic rather than Vandalic. It is mutually intelligible with Gothic as we understand it though, and suggesting it was in use among the Vandals.