Hi Art -- thanks for posting to History Hub!
To find public testimony before the House Un-American Activities Committee (HUAC) I recommend starting with the Cumulative Index to Publications of the Committee on Un-American Activities, 1938-1954 and the Supplement to Cumulative Index to Publications of the Committee on Un-American Activities: 1955 through 1960, both of which are available online through the Internet Archive.
In the Cumulative Index on page 365, I see that Granville Hicks testified in a hearing published as Communist Methods of Infiltration (Education) in February 1953 -- his testimony starts on page 95. The hearing is freely available online at the link via Hathi Trust.
The two Indexes also indicate that Hicks's name is mentioned several more times in other published hearings. You'll likely be able to find those hearing transcripts online through Hathi Trust, Google Books, or the Internet Archive. To use the Indexes, note the letters in bold and the page numbers next to an individual's name, then use the 'Key to Symbols' at the beginning of the Index to find the publication title.
It's also possible that Granville Hicks testified before the committee in executive session and / or that there is an investigative file on Hicks with the HUAC records held by the Center for Legislative Archives. To inquire about HUAC records, please email email@example.com. Although please note that due to the Coronavirus public health emergency, our building is currently closed and we do not have access to the original, unpublished records in the stacks during this time.