From Wilipedia — of course that’s not a perfect source, but it’s what I found quickly....
List of alleged returnees
Names of Guantanamo captives who are alleged to have “returned to the battlefield”
Maulvi Abdul Ghaffar — Had been a senior Taliban military leader prior to capture. Allegedly captured in Afghanistan in December 2001, was one of the twenty-three prisoners released from Camp Delta in late January 2004. After his release, he joined the remnants of the Taliban and was killed in a gunfight on September 26, 2004The official list of Guantanamo captives included two men with the same name, who remained in custody years after Maulvi Abdul Ghaffar had been reported to have been released, and killed in combat.[6]
Abdullah Mehsud — Reportedly captured in Afghanistan in December 2001 after surrendering to Abdul Rashid Dostum.
That he was ever been captured, and sent to Guantanamo has been challenged.[5]
Allegedly masterminded the kidnapping of two Chinese engineers in Pakistan’s South Waziristan region.
Allegedly returning to his position as an Al-Qaeda field commander.[12] One of the Chinese engineers died during a rescue mission, the other was rescued.[2]
Mehsud also claimed responsibility for the bombing at Islamabad’s Marriott Hotel in October 2004. The blast injured seven people, including a U.S. diplomat, two Italians and the Pakistani prime minister’s chief security officer. Mehsud was subsequently reported to have been killed in combat.
Mohammed Yusif Yaqub aka Mullah Shahzada — Reports of the release, return to the battlefield, and subsequent death in combat of Mullah Shahzada, while reported in the press, is always attributed to unnamed insiders. The official list of Guantanamo captives included a man the same name, Haji Shahzada who remained in custody years after the stories that Mullah Shahzada had been reported to have been released, and killed in combat. Haji Shahzada was one of the 38 captives whose Combatant Status Review Tribunal determined they had not been an enemy combatant in the first place.
On Monday, May 14, 2007, Pentagon officials, for the first time, tied the reports that “Mullah Shahzada” had returned to the battlefield to the name of one of the captives on the official list of Guantanamo captives, Mohammed Yusif Yaqub. According to Reuters summary of their testimony:
“Released May 8, 2003, he assumed control of Taliban operations in Southern Afghanistan and died fighting U.S. forces on May 7, 2004.”
Mohammed Ismail — First identified as a former captive who had returned to the battlefield in Testimony before Congress on Monday May 14, 2007. According to Reuters summary of their testimony:
“Released from Guantanamo in early 2004, he was recaptured four months later in May while participating in an attack on U.S. forces near Kandahar. When captured, Ismail carried a letter confirming his status as a Taliban member in good standing.”
Abdul Rahman Noor — First identified as a former captive who had returned to the battlefield in Testimony before Congress on Monday May 14, 2007. According to Reuters summary of their testimony:
“Released in July 2003, he has since participated in fighting against U.S. forces near Kandahar. After his release, he was identified as the man described in an October 7, 2001, interview with Al Jazeera television as the “deputy defense minister of the Taliban.”
Mohammed Nayim Farouq — First identified as a former captive who had returned to the battlefield in Testimony before Congress on Monday, May 14, 2007. According to Reuters summary of their testimony:
Released from U.S. custody in July 2003, he quickly renewed his association with Taliban and al Qaeda members and has since become “reinvolved in anti-coalition militant activity.”
un-named — Reports have circulated that one of the three children who was held for a year and a half, in Camp Iguana, and released on January 28, 2004, was subsequently captured, or subsequently killed in combat — accounts vary.[17]
As with “Mullah Shahzada” this information is attributed to unnamed insiders.
Accounts of when he was captured, or killed, vary.
Oliver North claimed that the released child was “Mullah Shahzada”.[21] North claimed that “Mullah Shahzada” was killed in combat weeks after his release. Mullah is an honorary title, meaning “educated man”. However the only schooling the three children held in Camp Iguana ever received was the lessons they received at the camp.[23][24][25] North’s account that a released child from Camp Iguana was killed in combat, weeks after his release, is at odds with the accounts of the journalists who interviewed the children during the months following their release.