I have a contrary view:
Geary was a member of that committee, and so he had to know that Frankie Pentangeli had survived and was being hidden away as a perjury trap for Michael. Geary was part of that trap:
When Michael first testified, he and Tom didn't know about Frankie--they thought Cicci was the highest-level witness the committee had against the Corleones. Geary asked Cicci a question that was seemingly helpful to Michael: "Did you ever get a direct order from him? Or were there always buffers?" "No, I never talked to him," replied Cicci. Michael relaxed: Since (it seemed) no one could say they got a direct order from him, he was free to lie under oath about the charges the committee leveled against him.
Then, bada- bing! The committee produced Frankie, who could corroborate those charges. Michael could be sent away on five counts of perjury--in large part because Geary had made him relax enough to lie under oath by asking Cicci that seemingly helpful question.
Some people here might believe that Geary wouldn't have knowingly tried to trap Michael--it'd be suicide. Certainly it would have been risky. But as we saw so many times in the film, people bent on revenge (like Carlo and Fredo) don't think logically--and Geary had every reason to want to avenge himself on Michael.