'Laid Back' first appeared in October 1973 and is vastly expanded for this release, while the live set has been unavailable for more than 30 years.
The great writer-guitarist, with wife Bonnie, made some of the best blues, rock and gospel-flavoured US music of their generation.
The titles include the band's self-titled debut from 1970, 'Eat A Peach' and the much-acclaimed live album, 'At Fillmore East'.
'At Fillmore East' cemented the band's live reputation for delivering incendiary southern rock.
Gregg was born on 8 December 1947 in Nashville, one year the junior of brother Duane, with whom he would co-found the mighty blues-rock institution the Allman Brothers Band.
They will be joined on the Prog In The Park stage by Polish outfit Riverside and Pain Of Salvation, with further artists to be announced.
The news comes hot on the heels of last week’s announcement that Foreigner will headline the festival's main stage.
As 'Brothers and Sisters' entered the album chart, its Dickey Betts song ‘Ramblin’ Man'' hit the Hot 100 to become the band's first major hit.
The song had both historical significance and huge poignancy in the pre-ABB days of Duane and Gregg Allman.
History has tended to overlook the festival that was bigger than Woodstock, Monterey or the Isle of Wight.
In the grounds of a stately home in Hertfordshire, the Bucolic Frolic culminated in some mighty southern rock.
Two critically-acclaimed albums from singer-songwriter John Hiatt, originally released by A&M, are set for special vinyl reissue on 26 January.
Former co-lead guitarist of The Allman Brothers Band, Dickey Betts, is to headline the annual Allman Brothers-founded Peach Music Festival 2018
Legendary record producer and Muscle Shoals studio pioneer Rick Hall, who worked with Aretha Franklin and many more has died aged 85
Macon, Georgia, have officially named 8 December Gregg Allman Day in perpetuity in honour of the late Allman Brothers Band frontman