The 38-year-old was Gillingham manager Andy Hessenthaler's player-coach at Dover Athletic.
"I heard nothing from Dover's new manager [Martin Hayes], so I thought I'd make his decision easier."
Southall became the fourth member of Dover's management staff to join Gillingham this summer, after Hessenthaler, assistant Ian Hendon and academy coach Darren Hare.
"When Andy called me to ask if I'd come back, Dover hadn't been in contact and I assumed their new boss would want his own assistant." said Southall.
"As far as I was concerned, the opportunity arose at Gillingham, and being a Gills man through and through, it was too good to turn down."
Source: BBC Sport
Source: BBC Sport