Hartline, 28, had started 69 games for the team since 2009, including every game last season. Gibson, 27, started five games last season.
The two moves clear $6.4 million in salary-cap space for the Dolphins, who were less than $3 million under their projected adjusted salary cap for the 2015 season.
Hartline, who signed a five-year deal in 2013, was due to count $7.35 million against the Dolphins' salary cap next season. Gibson, who signed a three-year deal in 2013, was set to count $4.26 million against Miami's cap.
Hartline posted back-to-back 1,000-yard receiving seasons in 2012 and 2013 before a drop-off last season, when he was limited to 39 catches for 474 yards.
Gibson caught 29 passes last season for 295 yards and one touchdown.
The Dolphins could also release or trade wide receiver Mike Wallace, who is due to count more than $12 million against the team's salary cap next season. The South Florida Sun-Sentinel reported Thursday that Wallace is not willing to restructure his lucrative five-year, $60 million deal, signed in 2013.
Wallace led the Dolphins with 10 touchdowns in 2014 but was unhappy not to be targeted more. Coach Joe Philbin benched him in the second half of the final game after an argument between the star receiver and coaching staff.
Jarvis Landry, who led Miami with 84 receptions as a rookie last season, is expected to move into a starting role. Receiver might also be a priority in free agency and the draft.
Information from The Associated Press was used in this report.