The midfield duo were among seven players offered new deals at the end of last season but they have dismissed the terms on the table and will now look for new clubs.
Guttridge and Harding both started May's League Two play-off final at Wembley as Town lost 3-0 to Bradford.
"I would like to place on record my sincere thanks to both players for their work over the last 18 months or so," boss Aidy Boothroyd told the club's official website.
"They have both played major roles in helping us avoid relegation the season before last and then getting to Wembley last season.
"We made both players offers at the end of last season, but both feel the offers are not what they are looking for and we fully respect that. We have shaken hands and we wish them both well and every success for the future.
"They are both good personalities and wherever their futures may take them, they go with our thanks and best wishes.
"Just as they will move on, we have moved on, we are in talks with our midfield targets and we are looking forward to the start of pre-season on Friday."
The news comes on the same day as Town announced the capture of versatile midfielder Ian Morris from Torquay.