The 7-foot Spanish forward said Chicago was a logical choice since "without doubt, it is one of the championship favorites in the East alongside Cleveland."
The 34-year-old Gasol, who departs the Los Angeles Lakers after six-plus seasons, said while the Spurs and Thunder were options, "nothing is guaranteed wherever you sign. I'm happy with my choice."
Gasol also expects newly acquired forward Nikola Mirotic, who was signed from Spanish club Real Madrid, to "make an immediate impact" with the Bulls.
The Bulls and Gasol agreed on a multiyear deal Saturday, when the four-time All-Star announced his decision on Twitter.
The Associated Press contributed to this report.