Gabbiadini, 32, has signed a three-year deal with The Cobblers after completing a move from Darlington for an undisclosed financial settlement.
The Nottingham-born striker is joining the tenth club in his career, during which he has made almost 600 appearances and scored 210 goals, with 24 in each of his seasons at Darlington making him the Division Three top scorer for the last two terms.
Former Leeds United youth player Forrester, 25, has also signed a three-year contract in a club record £150,000 deal from FC Utrecht, following a loan spell at the Sixfields Stadium in which he scored six times in ten appearances in The Cobblers' promotion campaign.
The Bradford-born hitman has also played for Auxerre, Grimsby and Scunthorpe during his career, which stalled after a promising start at Elland Road.
Plymouth Argyle midfielder Hargreaves, 27, has signed a two-year deal after completing his Bosman move to the club.
The Cleethorpes-born player started his career at local Grimsby Town, and has also played for Hull and West Brom, before making over 90 appearances in a two-year spell at Plymouth.
Chairman Barry Stonhill thanked the club's supporters for their contribution to the signings through season ticket sales, and said: "I'm very pleased with each of these signings."
Kevin Wilson, the Cobblers' manager, was excited by the signings, adding: "Chris is a wide left player who can give us good quality. He is strong and quick and can deliver telling crosses and I expect Marco and Jamie to get on the end of them.
"When you bring people of this quality in, people sit up and notice. These are all good, exciting signings for Northampton."
Gabbiadini added: "I had a few good offers but I want to look at things in the whole. I know Kevin and was impressed by the club behind the scenes. I'm happy to sign and it's a good setup here.
"I've got a lot of experience but I feel I've got a lot left in me. I want to play at the highest level as long as I possibly can. The squad here is very good and hopefully we can build on promotion."
Forrester, whose move to the club has been on and off a number of times over recent months, said: "I'm delighted. There have been a lot of negotiations but I'm delighted to be here and I can't wait for the start of the season.
"It's a great club, Kevin has been great to me, as has the chairman and the fans are superb. Some people think we're only trying to avoid relegation next season but we're aiming much higher than that."
Hargreaves added: "It feels great to join Northampton alongside such quality players. Kevin Wilson was a big factor in me choosing Northampton ahead some of the other offers I had. I'm very confident we can do well."
