~구만 = Incorrect but widely used form of ~구먼

~구먼 has same meaning and usage as ~구나 but

~구먼 is 하게체 (familiar speech level), used by older people, but not to children, in novels, and between adult male friends. 

기가 막히는구만 = I can’t believe this.

자네는 돈이 별로 없구먼! = You (younger man) have barely any money! 

유능한 젊은이 처럼 보이는 구만 = Seems like a capable young man