According to Japanese lip readers, his last words were "Koishiteru ne, ryoukai," which is primarily used to express the deepest form of love. Interestingly, Japanese people believe that this word shouldn't be used directly in manga, novels, or any media, as it would diminish the meaning of this beautiful term. This word is reserved for private use and is meant to convey a profound love, like "wanting to spend each and every breath with you" type of love.
Which makes sense why we never get to hear Gojo's last words to Geto.