Full disclosure, you asked a very similar question a day or so ago here at Ask Historians to which I said it was fictional and indeed it was from the novel. I'm afraid the answer hasn't changed since then or my further replies yesterday

Was it real? The historical texts give us no special weapon, Guan Yu's biography, compiled from the records of his own side by a former Shu-Han officer Chen Shou into said biography, only mentions an arrow as a specific weapon. An arrow that injures his arm. Wu commander Lu Su's biography does have them both meeting during the 215 war with a single sword as an agreed condition for both sides. Nor am I aware of any later source of history like Sima Guang's ZZTJ crediting him with a specific weapon.

I'm unaware of any Guan dao's being used in that era. For Guan Yu to keep one weapon his entire life would be unusual for the time, yet gets no mention in the texts for this unusual, and would imagine rather difficult given wear and tear combined with desperate flights over the years.

When scholars have written of Guan Yu they fail to mention it when discussing only the history. Li Teng does with the novel and Kam Louie uses the introduction of the blade in the novel as an example of the sexual motifs and themes around Guan Yu. Both make clear it is novel only, that there is no historical evidence of his using or having such a weapon.

It isn't in the historical texts, it goes against what we know of the era, academics say novel only and the first source that mentions Guan Yu wielding that weapon is a novel thousands years later that is full of, well, fiction. In that opening chapter alone: four named people who don't exist, it has two duels and officer deaths as part of the way the nature of war is changed by the novel as I touched upon in past thread, it has Liu Bei and co fighting in fronts they were not involved in. Gives Zhang Jue magical powers, He Jin has the wrong rank for that moment in time, fictional appearances, fictional oaths and weapons. Fictional meetings between Liu Bei and first Liu Yan then Dong Zhuo, a Turban revolt in a province known for the Turbans sitting that revolt out, Zhang brothers on the wrong front...

I'm aware there are claims a Taoist temple has the weapon, not been able to track much information about if the temple is even claiming it bar repeated Wikipedia rumours. However, you have a Guan Yu wields this weapon being something that isn't mentioned for over a thousand years and then only in a work of fiction. No academic work that I'm aware of backing it being history, the guandao not being mentioned in the three kingdoms era itself.

Guan Yu had no Guandao. The Green Crescent Dragon is a work of fiction adding to Guan Yu's legend. A temple that may or may not claim to have a weapon from a fictional novel doesn't override everything else

Sources:

The Records of the Three Kingdoms by Chen Shou, translated by Pei Songzhi

Comprehensive Mirror in Aid of Governance (the ZZTJ) by Sima Guang

Imperial Warlord: A Biography of Cao Cao 155-220 AD by Rafe De Crespigny touches upon weaponry of the era.

Romance of the Three Kingdoms by Luo Guozhong, translated by C.H. Brewitt-Taylor

Making the Guan Yu Cult: The Rise of Guan Yu in National Sacrifice, Buddism and Taoism by Li Teng

Guan Yu’s life after death: The religious and literary images of the Three Kingdoms hero Guan Yu by Jesper Timmerman

Sexuality, Masculinity and Politics in Chinese Culture: The Case of the 'Sanguo' Hero Guan Yu by Kam Louie can be read on jstor