Referring to PM Modi's speeches in parliament on Wednesday, he told reporters that it was clear that the prime minister was not interested in answering "fundamental questions".
Defence Minister Nirmala Sitharaman, he said, had first announced that she would inform the nation about the cost of purchasing Rafale aircraft but was now saying she could not do so as it was a state secret.
"Which of her two statements are correct," he asked.
The Congress President also posed a multiple choice question on Twitter over the reason behind Ms Sitharaman's change in stance.
Q. Why did the Raksha Mantri change her stance from :— Office of RG (@OfficeOfRG) 8 February 2018
I will reveal the price of the RAFALE planes in Nov 2017 to the price is a state secret in Feb 2018
B. To protect Modiji
C. To protect Modiji's friend
D. All of above #TheGreatRafaleMystery
Mr Gandhi said he had also posed three questions to PM Modi.
These included the aircraft's cost and "handing over" the deal to a businessman after "snatching" it from a public undertaking.
However, there was no reply, Mr Gandhi said.
"There can be only one answer and it is that there is something fishy about this," he said.