The two are about the same in their most common meaning of "to cross", but they have other lesser meanings too. 건너다 1) cross (a road, river, obstacle, etc). "길을 건너다". 건너다 2) skip an expected step in a routine (건너뛰다). "끼니(meal)를 건너다". 건너가다 1) same as 건너다 1). "길을 건너가다". 건너가다 2) move/go to a new place (in a major relocation like going overseas). "그는 일본으로 건너갔다". For the #1 meaning, the two words are practically the same, and can be used interchangeably in most cases.