In April, Alphabet CEO Sundar Pichai took an unusual step: merging two large artificial intelligence teams—with distinct cultures and code—to catch up to and surpass OpenAI and other rivals.

Now the test of that effort is coming, with hundreds of people scrambling to release a group of large machine-learning models—one of the highest-stakes products the company has ever built—this fall. The models, collectively known as Gemini, are expected to give Google the ability to build products its competitors can’t, according to a person involved with Gemini’s development.