The Wikipedia contributors did a fairly decent job of gathering what little information is available about Gugalanna.
The only text in the Electronic Text Corpus of Sumerian Literature (ETCSL) that mentions Gugalanna/Gud-gal-ana is
Inana's descent to the nether world,
a poem in which the goddess Inanna decides to visit the Netherworld.
When Neti, the chief doorman of the Netherworld asks her why she has come there, she answers
(quoted from the translation on ETCSL):
Holy Inana answered him:
"Because lord Gud-gal-ana, the husband of my elder sister holy Erec-ki-gala, has died;
in order to have his funeral rites observed, she offers generous libations at his wake -- that is the reason."
The manuscripts of "Inanna's Descent", also known as "From the Great Above to the Great Below" (from its first line of an older translation),
date from ca. 1900 – 1600 BCE (see Joshua J. Mark: Inanna's Descent: A Sumerian Tale of Injustice),
but the story may be older.
The text does not mention how Gugalanna died, unlike the Sumerian poem "Bilgames/Gilgamesh and the Bull of Heaven", which does not mention the name Gugalanna.
(None of the other Gilgamesh texts that render the episode about the Bull of Heaven use the name Gugalanna, i.e. the Hittite version and the Standard Babylonian version of the epic.)
Manuscripts of the Sumerian Bilgames/Gilgamesh poems date from the 18th century BC (Sallaberger: Das Gilgamesch-Epos, page 60), although the stories may be three centuries older.
Based on this, it seems hard to argue that the name "Gugalanna" represents an older tradition than the Sumerian Bilgames/Gilgamesh poems
(i.e. there is insufficient evidence to support the hypothesis that scribes stopped using the name Gugalanna during the period between the composition of "Inanna's descent" and "Bilgames and the Bull of Heaven").
Jeremy Black and Anthony Green thought that "[Gugal-ana's] name probably originally meant 'canal inspector of An'" and that therefore he "may have been identical with Ennugi".
(They write this in the entry for Ereskigal, since Gugal-ana doesn't have his own entry.
See Black, Jeremy; Green, Anthony: Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary. The British Museum Press, 1992.
An is the Sumerian word for "heave"; An or Anu is the sky god.)
However, Gwendolyn Leick thinks Gugalanna's name means "wild bull of Anu" and mentions that he "is quoted in some god-lists as the husband of Ereškigal"
(Gwendolyn Leick: A Dictionary Of Ancient Near Eastern Mythology. Routledge, 1998).
Leick also states that "was identified with Nergal by the time of the Old Babylonian period".
Since in a later tradition, Nergal became Ereškigal's (second?) husband (see "Ereškigal (goddess)" in Ancient Mesopotamian Gods and Goddesses),
the identification of Gugalanna with Nergal may be a result of this later tradition. [Note 1]
It is not clear where the identification of Gugalanna with Ennugi comes from.
Ennugi is mentioned in only one text in the Electronic Text Corpus of Sumerian Literature (ETCSL): A šir-gida to Nuska (Nuska B), which contains the following lines:
You make pleasing the offering table of Nintur. You issue orders for Ennugi. You …… life for the king. You …… over the single path of heaven and earth.
Conclusion: Based on the above, information that supports the claim that the Bull of Heaven's name was Gugalanna appears to be very scanty. In the sources listed above, there is no direct explanation of how Gugalanna come to be identified with the Bull of Heaven, except for the translation of his name as "wild bull of Anu".
Nergal became co-ruler of the Netherworld in the second millennium; the Neo-Assyrian poem "Nergal and Ereškigal" relates how he became Ereškigal's husband. Nergal is also mentioned in the Modernized Muss-Arnolt translation, namely in the flood story in Tablet XI and in another story in Tablet XII. However, in the corresponding passage in Tablet XI in the translations by Andrew George and Benjamin W. Foster, we find the name Errakal instead of Nergal. The events in the flood story predate the events in the rest of the epic, and the events in Tablet XII describe an alternative "death" of Enkidu, so Nergal being mentioned in the Muss-Arnolt translation does not allow us to draw conclusion about the identification of Gugalanna with Nergal.