Gohyaku-Jintengo (literally, five hundred is like a piece of dust in the infinite time frame) is a word that represents admiration for Siddhartha Gautama spending limitless time in the process of Jodo (completing the path to becoming a Buddha by attaining enlightenment), which is cited in the Nyorai Juryo-bon (the Infinite Life of Buddha Chapter) of Hokekyo (the Lotus Sutra). More precisely, it should be referred to as 'Gohyakuoku-Jintengo' (literally, fifty billion is like a piece of dust in the infinite timeframe).
In the Nyorai Juryo-bon, the 16th Chapter of Hokekyo, the following description is found:
The present Siddhartha Gautama left the Siddhartha (Gautama) Kingdom and his Gaya-jo Castle not so long time ago and wished that he could acquire Anokutara sanmyaku sanbodai (supreme perfect enlightenment, the unsurpassed enlightenment of a Buddha) while meditating in a training hall.'
But it has been muryomuhen hyakusenman no kunayutako (my lifespan should be limited, even to the extent of hundred thousand kotis of nayutas of kalpas, may I not attain perfect Enlightenment) since I became Buddha.'
The speaker continues, 'for example, suppose that the Sanzen Daisen Sekai (the universe consisting of 1,000 exponent 3 (=1 billion) Buddhism-style galaxies), with each galaxy measuring gohyaku-senman-oku nayuta asogi (five hundred, a thousand, ten thousand, a million nayuta asamkhya) was fallen apart to pieces of dust.'
If a person passes an Eastern (Buddhism-style) galaxy measuring gohyaku-senman-oku nayuta asougi go--in other words, if a person collect one piece of dust--and continues to go eastwards until collecting all the pieces of dust, is he able to know the number of (Buddhism-style) galaxies in the universe or not?"
A similar description is found in the Kejoyubon, the 7th Chapter, of the Lotus Sutra.
For example, suppose the all the seeds (a metaphor for Buddhism-style galaxies) in the Sanzen Daisen Sekai are ground to ink, even if you visited 1,000 Eastern galaxies, you will only draw a dot.'
The size of the dot is similar to a piece of dust.'
This anecdote in the Kejobon is called Sanzen-Jintengo (literally, thirty thousand (in actual sense, 1,000 exponent 3) is like a piece of dust in the infinite timeframe). Meanwhile, the phrase 'gohyaku-senman-oku nayuta asogi' of the Juryo-bon ((one of) 'honmon,' or primordial later 14 chapters of the Lotus Sutra) is called Gohyaku (oku)-Jintengo (literally, fifty billion is like a piece of dust in the infinite timeframe), suggesting (the process of Siddhartha's becoming Buddha is far more prolonged process than Sanzen-Jintengo in the Kejoyu-bon ((one of) 'shukumon,' or the first 14 chapters of the Lotus Sutra).
In the Lotus Sutra, Siddhartha's enlightenment is described as follows: 'It has been muryomuhen hyakusenman no kunayutako since I became Buddha.'
If the descriptions in the Lotus Sutra can be taken for granted, the Gohyaku-Jintengo is nothing but a metaphor and is not the time when Siddhartha became Buddha. The Sanzen-Jintengo in the Kejoyu-bon is also nothing but a metaphor.
Nichiren, however, added the connotation of Kaigon Kennon (Siddhartha is not only a real person who became Buddha but also a true Buddha who has been saving the public since the infinite time of Gohyaku-Jintengo) to Gohyaku-Jintengo, stating in his books such as "Book of the Realm of Siddhartha" that 'this Shaba (one of galaxies in the Sanzen Daisen Sekai, where our humans live) world has been the realm of Siddhartha Bodhisattva for past Gohyaku-Jintengo years.'
With this, it became common for Gohyaku-Jintengo to be interpreted as the time when Siddhartha really achieve enlightenment.
Incidentally, in general, it is said that Siddhartha was born in India and achieved enlightenment under the Bodhi Tree.
This is called 'Gaya Shijo' or 'Shijo Shogaku.'
The Hokekyo, however, confesses that Siddhartha is not an person who was born in India and achieved enlightenment under the Bodhi Tree but a Buddha who had achieved enlightenment in the infinite remote past. This is called Kuon Jitsujo.