Gregg Allman's most visible contribution to rock music is as lead singer, organist, and songwriter within the Allman Brothers Band, founded by his brother Duane (d. 1971) in 1969. He has never threatened to eclipse the band that carries his family name, but he has found occasional success and popularity with his solo work, which is distinctly different, more soulful and less focused on high-wattage virtuosity.
Allman's instrument is the organ, and he is most effective, when he is in top form, as a singer. His first instrument, ironically enough, was the guitar, and he took it up before his older brother Duane did. But Duane learned it better and quickly eclipsed Gregg.