Nottingham Forest will pay around £12.75m to sign Mangala, who played 29 times for Stuttgart last term.
“I had a really good season last season – full of confidence, full of experience,” Garner told the PA news agency. “The experience I didn’t have, I feel that now I do have. “I played pretty much every single game, played well and ended up winning promotion. For me, it’s probably the perfect loan.