반가운=glad: adjective 반가움=gladness: noun

Both are correct but the usages are different. Both are from 반갑다. 반갑다 means "glad, happy, pleasant, welcome " 반가운 is composed as 반갑(다) + (으)ㄴ (으)ㄴ is a suffix for making a determiner Generally a word of the declension with (으)ㄴ is not listed in dictionary. 반가움 is composed as 반갑(다) + (으)ㅁ (으)ㅁ is a suffix for making a noun ex) 반가운 소식 means "happy news" 반가움 means "delight"