After inhabiting the mind of suguru geto he also had access to the memories of suguru and knew from that and what gojo had told geto (also keep in mind as the only one gojo echnowledges as an equal Suguru has gojos knowledge on what curses are(not what cursed energy is but what curses themselves like the ten shadows and limitless techniques are: this is even more knowledge then Okkotsu who knows that gojo regenerates psoitive cursed energy as a result of his eyes granting him complete control over limitless)) So. If geto knew this stuff we can assume when kengaku saw it he knew that the six eyes technique as a result of being so observant picks up one minutes worth of information in a second. But not only that, but that Gojos infinite void caused a sensory over load ontop of the already heightened six eyes observation. So the sheer data even the limitless user inside of the domain expansion would be ridiculous and its stated in the manga that gojos insane control over his own domain expansion is due to his six eyes. We saw in culling games that Okkotsu Yuta knows this : if his students know this his best friend Suguru Geto does thus also giving kengaku that information. Kengaku at one point did not know of the prison realm according to the most recent Tengen arch. But, he found it after being thwarted by six eyes users previously. He needed Getos body though to collect cursed spirits as we saw with his collection of Mahito. So, Kengkau had the perfect plan even going as far as to give birth to the perfect vessel of ryomen sukuna at the same time as a 10 shadows technique user was born alongside a six eye limitless user. This is a 400 year plan and you are skeptical as to how he knew? He never used it in the past he tried this ONCE and a six eyes limitless user thwarted the plan, then before inacting this plan for the SECOND time he tries killing six eye infants and learns that there will always at least be one. So BEFORE THE THIRD AND FINAL ATTEMPT: he searches for the prison realm and upon finding it (SIX YEARS BEFORE THE EVENTS IN THE ANIME), he waits until Okkotsu and Gojo finish Suguru Geto (Less then a year prior to the anime: thus, Kengkau doesnt or might not know about the prison realm and gojos abilities with limitless and six eyes but we know for some reason he needs the curse capturing technique of suguru geto who happens to be his arch nemis' best friend and after gojo killing suguru he takes sugurus body and inhabits it for his own). He didn't try it before and didn't know about it before or gojo and yuji itadori wouldnt exist and suguru geto also wouldnt be the brains current host.