给 and 合 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.给 /*[k](r)[ə]p/ 合 /*kˤop/, /*m-kˤop/
give; by, for; to; for; for the benefit of; to give; to allow; to do sth (for sb); (grammatical equivalent of 被); (grammatical equivalent of 把); (sentence intensifier)
