Toney has been rewarded for his fine start to the season with five goals in seven Premier League games.
"We have been tracking him for a long time," said Southgate. "I had a good chat with (Brentford manager) Thomas Frank about him last season.
"His form and his qualities - not only his goalscoring but his technical ability, build-up play and aerial prowess - gives us a different threat to some of the other players we could pick."
The Three Lions travel to Italy and host Germany in the Nations League later this month, aiming to make amends for a dreadful start to the competition.
Southgate's men have taken just two points from their opening four games in the group, including an embarrassing 4-0 home defeat to Hungary last time out.
Other than the inclusion of the in-form Toney, Southgate has remained loyal to the core of the England team that reached the Euro 2020 final.
Manchester United duo Harry Maguire and Luke Shaw are included in the 28-man squad despite losing their place at club level under Erik ten Hag.
Kalvin Phillips has also found game time hard to come by since moving to Manchester City, while Chelsea's Ben Chilwell has only recently returned from a long-term injury.
“Clearly, we have a number of players including Ben Chilwell, Luke Shaw, Harry Maguire and Kalvin Phillips who are not playing a lot of minutes with their club," added Southgate.
"It is not ideal, but we feel they have been, and can be, important players for us. It is not a perfect situation but there is still a lot of football to be played before Qatar."
Another two United players remain out of the squad as injury put paid to Marcus Rashford's hopes of a recall and there is no place for Jadon Sancho.
Tottenham's Eric Dier is rewarded for his consistent form under Antonio Conte with a recall, while Chelsea's Ben Chilwell returns after a lengthy injury absence.
First-choice goalkeeper Jordan Pickford misses out with a thigh injury.
