"The two beneficiaries of the business dealings of Amit Shah's son Jay are Narendra Modi and Shah. Achche din aa gaye unke (good days have arrived for them)," Mr Gandhi, who is on a three-day visit to Gujarat, said at a public meeting.
"But people need not worry... elections are round the corner, and once the Congress comes to power, 'achhe din' will truly come for the common man," he added.
News portal 'The Wire' had recently reported that a firm owned by Jay Shah saw a huge rise in turnover after the BJP came to power at the Centre in 2014.
Mr Gandhi, who has been repeatedly attacking the two on the issue, said the company was formed six to seven years ago but its fortune started soaring after Modi became the prime minister.
"Modi ji came (to power in 2014) and launched 'Start up India', 'Make in India'. He then introduced demonetisation and GST. These destroyed small businesses and farmers. From this fire, one company comes up. It was nothing in 2014, but in a few months, the company grew and its turnover rose from meagre Rs 50,000 to Rs 80 crore," he said.
Referring to PM Modi's remark ahead of the Lok Sabha poll that he would prefer to become the "chowkidar" (watchman) of the country's wealth and not the prime minister, the Congress leader questioned his silence over the "theft".
"Theft has taken place in front of the eyes of the chowkidar. But you are silent. The question is are you a watchman or are you a partner?" he said.
Mr Gandhi, seeking to revive the Congress's fortunes in Gujarat ahead of elections later this year, also visited a temple and fed grass to a cow, in gestures loaded with symbolism.