Jakhar polled 4,99,752 votes while his BJP rival Swaran Salaria polled 3,06,533, poll officials said.
Aam Aadmi Party nominee Major General (Retd) Suresh Khajuria was at the third sport with 23,579 votes.
The seat had fallen vacant after the death of BJP MP Vinod Khanna in April this year.
The poll was seen as a prestige issue for the ruling Congress which won the assembly polls six months ago.
The Congress won this seat after the 2009 Lok Sabha poll when party candidate Partap Singh Bajwa had won this seat by defeating BJP candidate Vinod Khanna.
Also Read
"With this victory, people have reaffirmed faith in the leadership of Amarinder Singh," said Jakhar.
"The BJP must read the writing on the wall now," Jakhar said and added that the "People have rejected the BJP and also shown mirror to (their ally) the Akalis".
"Now, I think this (Gurdaspur win of Congress) will be the beginning of disintegration of SAD because nobody has seen Prakash Singh Badal for the last six months. The new leadership under Sukhbir Singh Badal is on the verge of disintegration. I think the entire SAD is going to be reformed under some new leadership, I believe," Jakhar said.
"The massive win for Congress candidate Sunil Jakhar shows the people's complete denunciation of the corrupt and immoral BJP and SAD," the chief minister said, adding that it also underlines the "political annihilation" of the Aam Aadmi Party (AAP) in the state.
"Jakhars victory comes as a strong rebuttal to the persistent Akali efforts and campaign of the past six months to undermine the work being done by his government in the state," he added.
"For AAP, this election marks the end of its political ambitions in Punjab," Amarinder Singh said, pointing out that the dissolution of the Pathankot and Gurdaspur units of AAP a day before the declaration of the results showed that the party had conceded defeat.
Meanwhile, Both the BJP and the AAP accused the Congress of misusing official machinery to win the bypoll.
Punjab BJP Secretary Vineet Joshi alleged that the Congress misused the official machinery in the bypoll
AAP candidate Maj. Gen. (Retd) Suresh Khajuria also accused the Congress of using "undemocratic means" in the bypoll.
"Ruling party used undemocratic means in these elections. People were scared and youth was almost absent in the bypoll. If they (Congress) win then that victory will not be a respectable one," Khajuria alleged.
Disclaimer: No Business Standard Journalist was involved in creation of this content