The Brazilian admits it was hard to say goodbye to the friends he made at the Allianz Arena after moving over the summer ahead of being reacquainted with his former team-mates
The Brazilian spent two years at the Allianz Arena and was part of the side that lifted the treble last season, before moving on in a €17.5 million deal as the club brought in Mario Gotze and Thiago Alcantara.
Gustavo will come up against his former team-mates on Saturday having served a suspension and he revealed the strength of the bond between himself and several of his former team-mates.
The 26-year-old told Bayern's official website: "Dante and Rafinha are two friends that I've gained for life from Bayern. But not only them, also Franck Ribery, David Alaba and many others. We were always a very good group.
"We often played against each other in practice matches in training and they were competitive, but this of course will be very different."
Bayern have made a promising start to life under Guardiola and lifted their first trophy under the former Barcelona coach when they beat Chelsea on penalties in the Uefa Super Cup in August.
And Gustavo has backed the Bavarians to continue their dominance both domestically and in Europe.
"Bayern are the best club in the world, you can definitely say that after last season," he added.
"Each opponent will try to beat them but they have a lot of quality and can do anything if they stick together again."