I read at some point Shakuni had 100 brothers and were arrested by Dhritarashtra, because of hiding first marriage of Gandhari with some tree or something and Dhritarashtra feed them with only one peice of rice in plate for each member. So by sacrificing that pieces of rice to Shakuni all of them died?
I'm seeking is this true or just a gossip? but not why it happened and Gandhar is the present Afghanistan?
It is just a folklore and is not present in any scriptures of Hinduism. Dhritarashtra didn't arrest Shakuni and his family.
The story you told is present across internet 1, 2. You can observe that they are not quoting any scriptures and are clearly saying that the story is not present in any Hindu scripture.
We can prove the falsity of the story with the help of the fact that Suvala, father of Shakuni, attended the Rajasuya sacrifice performed by Yudhishthira.
Vaisampayana said,--"the ever-victorious Nakula, the son of Pandu, having reached Hastinapura, formally invited Bhishma and Dhritarashtra. The elder of the Kuru race with the preceptor at their head, invited with due ceremonies, came with joyous hearts to that sacrifice, with Brahmanas walking before them. And, O hull of the Bharata race, having heard of king Yudhishthira's sacrifice, hundreds of other Kshatriyas acquainted with the nature of the sacrifice, with joyous hearts came there from various countries, desiring to behold king Yudhishthira the son of Pandu and his sacrificial mansion, and brought with them many costly jewels of various kinds. And Dhritarashtra and Bhishma and Vidura of high intelligence; and all Kaurava brothers with Duryyodhana at their head; and Suvala the king of Gandhara and Sakuni endued with great strength; and Achala, and Vrishaka, and Karna that foremost of all charioteers; and Salya endued with great might and the strong Valhika; and Somadatta, and Bhuri of the Kuru race, and Bhurisravas and Sala; and Aswatthama, Kripa, Drona, and Jayadratha, the ruler of Sindhu; and Yajnasena with his sons, and Salya that lord of earth and that great car warrior king Bhagadatta of Pragjyotisha accompanied by all Mlechcha tribes inhabiting the marshy regions on the sea-shore; and many mountain kings, and king Vrihadvala; and Vasudeva the king of the Paundrayas, and the kings of Vanga and Kalinga; and Akastha and Kuntala and the kings of the Malavas and the Andhrakas; and the Dravidas and the Singhalas and the king of Kashmira, and king Kuntibhoja of great energy and king Gauravahana, and all the other heroic kings of Valhika; and Virata with his two sons, and Mavella endued with great might; and various kings and princes ruling in various countries; and, O Bharata king Sisupala endued with great energy and invincible in battle accompanied by his son--all of them came to the sacrifice of the son of Pandu. And Rama and Aniruddha and Kanaka and Sarana; and Gada, Pradyumna, Shamva, and Charudeshna of great energy; and Ulmuka and Nishatha and the brave Angavaha; and innumerable other Vrishnis--all mighty car-warriors--came there.
[Section 33, Rajasuyika Parva, Sabha Parva, The Mahabharata]
According to the folklore stories mentioned over internet, Shakuni's father was dead and hence cannot attend the sacrifice with Shakuni. Therefore, the stories are not true according to the Mahabharata.
Shakuni and his family members were not arrested because Shakuni made no objections in giving Gandhari to Dhritarashtra.
It is mentioned in Sambhava Parva of Adi Parva, Mahabharata.
Soon after Bhishma heard from the Brahmanas that Gandhari, the amiable daughter of Suvala, having worshipped Hara (Siva) had obtained from the deity the boon that she should have a century of sons. Bhishma, the grandfather of the Kurus, having heard this, sent messengers unto the king of Gandhara. King Suvala at first hesitated on account of the blindness of the bridegroom, but taking into consideration the blood of the Kurus, their fame and behaviour, he gave his virtuous daughter unto Dhritarashtra and the chaste Gandhari hearing that Dhritarashtra was blind and that her parents had consented to marry her to him, from love and respect for her future husband, blindfolded her own eyes.
Sakuni, the son of Suvala, bringing unto the Kurus his sister endued with youth and beauty, formally gave her away unto Dhritarashtra. And Gandhari was received with great respect and the nuptials were celebrated with great pomp under Bhishma's directions. And the heroic Sakuni, after having bestowed his sister along with many valuable robes, and having received Bhishma's adorations, returned to his own city.
