Gasol said Tuesday that, after speaking with Bulls players Derrick Rose and Joakim Noah, "instinct told me to pick Chicago."
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.
Gasol Chooses Bulls Over Spurs, Thunder
ESPN's Marc Stein explains why the big-city atmosphere in Chicago was a big draw for Pau Gasol.