I believe that the second syllable is stressed - 감 /사/ 합니다 yo‎u can check the prononciation from natives here: www.forvo.com/word/감사합니다‎ ^^ in the phonetic spelling it looks like [kɑːmsaɦamnida], where [ː] marks not the emphasized syllable but the long vowel [ɑ]. you can also notice that the correct sound for ㅎ is [ɦ] (and not [h]). sometimes, if the word is pronounced quickly, you barely hear consonants [ɦ] and [n]. to add even more information - in Korean you pronounce syllables while breathing in or breathing out. I'm guessing that in this word it would be: 감 (breathing out) 사 (breathing out + most stressed) 합 (breathing in) 니 (breathing out) 다 (breathing out + stressed, but less stressed than 사)

its 감 -사 thank you 합니다 is used in honorific expression.