The oldest usage of that name was by a dynasty called Gurjara dynasty (not to be confused with Gujjar caste) of Bharuch (580 CE - 738 CE) [https://en.m.wikipedia.org/wiki/Gurjaras_of_Lata]
Origin of Gurjara - The Land
Presumably, after migration of this family to Patan, Mehsana, Palanpur – this area of SouthEast Rajasthan & North Gujarat came to be known as Gurjara, which remained so between 6th century to 12th century, characterized by the belt comprising (Check Pic 1 in order from left to right) Patan, Bhinmal, Jalore, Mandore, Didwana, Rajore, and Bayana.
Hence the usage of Gurjara as a Toponymic term carried by local Brahmins, Jains, Banias & Rajputs whenever they migrated from this region.
The first mention of the term Gurjara referring to this region was in Jain monk Uddyotana Suri’s book Kuvalayamala Kaha in 778 AD who also gave an exhaustive insight into the geography of the region and description of the various communities in it. He mentions the Brahmins, Jains, Kshatriyas (Rajputs), Bhils but not any Gujjar caste.
Imperial Pratihars of Malwa become Gurjaresh
The Imperial Pratihars were rulers of Avanti (present-day Malwa) ruling from their capital at Ujjain from 730 CE. The first ruler of this dynasty Nagabhata Pratihar had conquered Gurjara which is evident from the
Gallaka inscription dated 795 CE which mentions that Nagabhata I as the one who had acquired victory over the “invincible Gurjaras” and obtained fame. This is hard-hitting evidence that the Pratihars prior to it, were not even connected to the term “Gurjara”. The “invincible Gurjara” mentioned here, referred to Gurjara dynasty of Lata . After defeating the Gurjara ruler ie Chapkota Rajputs of Annhilvada Patan, the Imperial Pratihars of Malwa ruled the region through their cadet branches. They never identified themselves with the term “Gurjara” in any of their inscriptions, yet where often called Gurjaranaresh (lord of Gurjara) in inscriptions of their Rashtrakut rivals.
Origin of Bargurjar Rajputs, a subclan of Imperial Pratihars
Pic 2 & Pic 3: The cadet branch of Pratihar Rajputs that stayed on in Bhinmal gradually became Deval Pratihar Rajputs while another splinter group that migrated out of Bhinmal to Rajor (near Alwar), became the first to use this Toponymic term to identify itself as “Gurjara-Pratihara”. As per Rajor inscription dated 960 AD, Manthandev Pratihar identified himself as “Gurjara-Pratihar”, who eventually became the progenitor of the Bargujar Rajputs, which as per all historians is another of Pratihar or Parihar Rajput subclans. The phrase "BarGujar" बड़ गुजर translates to Big Gurjar, underscoring their political superiority over all other Gurjara cognomen castes. Check Pictures 4 & 5 to see the Inscription details.
Gurjara Brahmins - Shrimali, Gour, Pareek
The Shrimali Brahmins & Pareek brahmins of Rajasthan and Gujarat were often called Gurjara-brahmins due to their origin here. In fact, one of the chief towns of ancient Gurjara was Bhinmal, which back in the day was famous as Shrimal. The Shrimali Brahmins are hence called so due to their origins in Shrimal / Bhinmal. Together with the Pareek, Gaur and Vyas Brahmins, the Shrimali brahmins are called the Gurjara-Brahmins, a term which has not been renounced until today. Check here: http://www.pareeksamaj.com/…/brahmin…/gurjar-gour-samaj.html
Similarly, the Jains who migrated from this region became Gurjara Jains, as against Oswal Jains from Osian (near Jodhpur). Check here:
Origin of Gujar Caste
The region of Gurjara as opined by Uddyotana Suri has always been home to a large number of pastoral communities - the Rebaris, the Vanjaras, and Gaddiya Lohars. Airavat Singh opines that on drying up of the Sukri river in the 13th century a large number of pastoralists of this region who spread out to other parts like eastern Rajasthan, west UP, Himachal, Punjab, and J&K eventually becoming the Gujjar caste of today.
The mixed origins of the pastoral Gujjar caste of today is evident when one finds Rebari clans like Khatana, Rajput clans like Chauhan, Bania clans like Bansal, Khatri clans like Chopra and brahmin clans like Nagar in them.
Hence Gojri, the language unique to Gujjars and spoken by Gujjars across Himachal, J&K, and Pakistan has been unanimously classified as an East Rajasthani dialect by famous linguists like George Abraham Grierson, a classification that remains undisputed till today. This would not have been possible if the Gujjars of all these regions did not have an East or Southeast Rajasthani origin.
K M Munshi, Gurjar Sabha & modern Gujarat State
After the 13th century, the word Gurjara lost its usage until it was picked up by 20th-century Statesmen like KM Munshi, a Vyas brahmin, who even established the Gurjara Sabha, of which both Jinnah and Gandhi were important speakers in 1915. The aim of it was to promote Gurjara Bhasha (not Gojri of Gujjar caste but Gujarati of Gurjar people ). The word regained its usage from there to refer to not just that specific region that was once called Gurjara, but for the entire region where Gurjara Bhasha or Gujarati was the dominant language.
It was hence the intellectual seeds sowed by K M Munshi, a Gurjara Vyas brahmin himself, in the early 20th century and the laborious work by Indulal Kanaialal Yagnik led Mahagujarat Movement which caused splitting of Bombay State, followed by the formation of Gujarat on 1 May 1960.