I went into this issue in some detail in this post not two weeks ago. Briefly put, the evidence for Gilgamesh himself is ambiguous and involves sources which are not uniformly reliable, but we have independent evidence (from inscriptions) of another ruler from the same time who was mentioned in one of the early Sumerian poems about Gilgamesh and was previously also thought to be a mythical figure. This indicates, at the very least, that Sumerian scribes had historical information from that period available to them when the early Gilgamesh stories were composed.

So Gilgamesh, of the eponymous epic, appears on the Sumerian King List as the King of Uruk around the year 2600 BCE and is said to have ruled for a 126 years.

What gives credence to his existence as a historical figure is another name on the Sumerian King List. En-me-barage-si, the king of Kish in around 2600 BCE who ruled for 900 years and the man "who made the land Elam submit." Now En-me-barage-si is the first king on the King List we can verify by an independent historical source and an independent archaeological source. He appears on the Tummal Inscription which mentions him as one who build the temple of Enlil in Nippur.

"Enmebaragesi, The king in this very city (that is Nippur), built the House of Enlil, Agga the son of Enmebaragesi, made the Tummal pre-eminent."

At the site of Nippur two alabaster vase fragments have been recovered that bear his name and can be dated to around 2600 BCE. This evidence indicates that En-me-barage-si was indeed a historical figure.

Now En-me-barage-si was the father of a Aga of Kish who appears on the Tummal Inscription and in the in the Epic of Gilgamesh where he wars with Gilgamesh. So though there is no direct evidence for Gilgmash himself the reasonably verifiable existence of other characters in the epic indicate that it is not pure mythology and is based in some historical understanding.

Where did you read this, out of curiousity? I've never heard that.