As mentioned in andejons’ answer, there is not a lot of material in the Gothic language. Apart from Wulfila's Bible translation on the Wulfila project (where you can also find a calendar fragment, the Gothic signatures of the Naples and Arezzo deeds and the Skeireins fragment, a part of a commentary on the gospel of John), there is another website, www.gotica.de, which apart from the aforementioned fragments also contains some other smaller fragments in Gothic. These texts are useful to study the Gothic language and how the Goths interpreted the Bible, but we can learn very little about their mythology from these texts. The problem is that the Goths already had become Christians before most written sources mention anything about them, so we do not know much about their pre-christian stories and beliefs.
However, in Latin we do have a text known under the title De origine actibusque Getarum or Getica, by Jordanes. It mixes earlier Roman and Greek sources with oral Gothic sources, and it represents both the mythological and historical background of the Goths. However, a lot of information is drawn from sources that write about peoples who were not actually Goths but other peoples with similar names or living earlier in the same areas that where later inhabited by Goths. A Latin edition can be found online here and (together with an English translation and notes) here. A more recent scholarly edition of the Latin text with elaborate notes and a side-by-side Italian translation was published in France in 2017.
In Old-Norse, we have the Hervarar saga ok Heiðreks, in which we find a poem commonly known under the name of Hlǫðskviða, about a battle of the Goths with the Huns. This poem may or may not reflect an earlier Gothic tradition about the same events. An Old-Norse version can be found here and (together with an English translation and notes) here. You may also want to look at Wikipedia for some basic background information.
You might be interested to know that, apart from this, in later times myths about the Gothic kings Ermanaric (also mentioned in the Getica) and Theoderic (a historical ruler of the Ostrogoths) started to appear elsewhere in Europe, for example in the Latin Annals of Quedlinburg, the Old High German Hildebrandslied, the Old English poems Waldere, Deor and Widsith, the Old-Norse Þiðreks Saga and some texts in the Poetic Edda, as well as in the Middle High German Heldenbücher and the Niebelungenlied (amongst others). But note that these are no Gothic stories, they are stories of a much later date about Gothic heroes who lived several centuries earlier.