https://en.wikipedia.org/wiki/Chinese_gods_and_immortals#Gods_of_Indian_origin
格薩爾 Gésàěr or 格斯爾 Gésīěr (+ 王 wáng or + 神 shén) — "King Gesar" or "God Gesar", an interethnic culture hero of a namesake epic poem, worshipped by Tibetans, Mongols, Han Chinese and other ethnic groups. He derives from Divus Caesar of ancient Rome through a cultural transmission from the peoples of Central Asia.
Oh my...
I must say, of all the things I've heard about the Gesar myth, I've never heard that he's been worshipped by Han Chinese. While it's not inconceivable that the Han might do so (the legend in various forms certainly is practiced as far as Buryatia on Lake Baikal in Russia, in Mongolia, Tibet obviously, and in Bhutan) I find it rather hard to believe. Mainly because there is such a rich breadth of native Han Chinese story-telling, and the Epic of King Gesar reeks of high land steppe culture.
More documentary evidence, however, tells me that there was an active effort to suppress the Gesar epic, probably as as result of it being identified with Tibetan culture and possibly as a rallying cry against Chinese oppression (the story is about a semi-diving King who rises from obscurity to defeat demonic forces of oppression in four directions). My copy of "Chinese Literature : Fiction Poetry Art, Spring 1986" includes a short article on "Cultural News" in the back:
Emergency Measures in Tibet to Save King Gesar
Stories about the legendary Tibetan hero King Gesar, sung or recited by folk artists, have become the longest epic in the world and are extremely popular in the region. The epic tells of the king of an ancient tribe who fought for the happiness of his people against monsters and tyrants and supported the weak and the poor. The extraordinary length and thorough Tibetan nature of the epic have long excited the atttention of scholars at home and abroad, yet during the "cultural revolution" some manuscripts in private hands were burned, and performance was forbidden. The verbally transmitted epic was on the verge of being lost. The effort to save it was begun in 1980 by a special group set up in Tibet. The six teams sent out discovered among the local population 40 minstrels who knew the stories, and over 50 versions were recovered. The teams also visited battlefields and antiquities associated with Gesar. So far over 170 stories have been collected. Allowing for repitition, says the group, this still amounts to 80-100 stories, 10 million words in all. The group's main work is finding artists and recording them. An 80-year-old story-teller named Zhagba, who began telling the story at the age of nine, is now in Lhasa singing and reciting for the group. He claims to know 43 stories, of which 22, about four million words, have already been recorded.
The "historical artifacts" referred to in the blurb, AFAIK, have not been dated, authenticated, nor have any attempts been made in that direction. Admittedly, I'm not all that up to date on Gesar scholarship, though it is an area of interest to me. I'll take a look at my library and see what I can peruse. Last I checked, however, modern scholarship has pointed to the Gesar story originating from an historical figure who lived approximately in the 8-9th Centuries in Kham (eastern Tibet). This would put him way after Caesar.
I'm not entirely convinced of that. The events described in the poem (even the shortened version I own) are fanciful even by Tibetan standards (and this is coming from a country whose official history c.1670 includes the description of semen entering the eye of their first king giving birth to the nation). That said, I'm even less convinced of the Caesar theory, and from what I've read about the subject, most modern Tibetologists find the theory to be silly at best.
The only evidence for it seems to be that Tibet was well connected, with Christian monks visiting the country at least since the 1200s. And with the Gesar epic being largely oral (like so much of Tibetan culture itself) it's almost impossible to prove (or disprove).
The evidence against it, however, seems to be a little stronger. Not least of which being that Gesar is said to have ruled a Kingdom in eastern Tibet (whereas, we would expect Tibetan literary culture to place a great mythical king in either Lhasa, south-central Tibet, or Mt. Kailash [to be close to the gods] in western Tibet). Even still, it's a mystery to me how Tibetans got turned around and places Caeser in the east of their country.
(Remember, most culture is preliterate. We're talking about a time period when most people could tell their directions with their eyes closed, especially in a country that is semi- or fully nomadic.)
The only evidence, I can tell, that associates Gesar with Caesar seems to be the slight similarity in names. I can't locate any similarity in their stories. Gesar actually has way more in common with Beowulf than Caesar. But, IMO, that has less to do with a common origin, and more to do with how mythology works. But that's a story for another time.
Without digging too deep into the hypothesis valley, the association of Gesar with Caesar probably has a lot to do with what I tend to refer to as "Shangri-laism," the tendency to idealize Tibet/the Himalayas as a place of perfection and spiritual supremacy. This mindset has roots in Orientalism, but reached its peak among the Theosophical movement from the late 1800s to the post-war era. The holy land of Theosophy was Tibet. The Theosophical Society's founder, Madame Blavatsky even claimed she had secret knowledge transmitted to her via Tibetan sages she called "the Mahatmas."
A common theme in Shangri-laism tends to be the merging of east and west. James Hilton, in his novel Lost Horizon where the name "Shangri-la" was invented, puts western Christian values, embodied in the sacred Duke of Luxembourg, in a western-style castle at the top of the Himalayas. There's hardly an Asian, and nary a Tibetan to be seen in the whole story. Other Theosophists, spiritualists, and self-proclaimed scholars have sought to unscientifically prove that Jesus' lost years were spent in India or the Himalayas. One of the most famous being "The Life of Saint Issa" by Nicolas Notovitch, which proved that Jesus spent his formative years in Ladakh, learning from Buddhist monks. Today this trend continues with books like (admittedly good...) Lamb: The Gospel According to Biff, Christ's Childhood Pal.
There seems to be the same sort of phenomenon going around with this Caesar/Gesar connection. And my current understanding is that it comes from the same source of misunderstanding. (For more on this topic of Shangri-laism, see Donald S. Lopez's Prisoners of Shangri-la.)