嘎嘎 is an onomatopoeia that imitates the sound of ducks quacking or a person's loud, hearty laughter. It can be used as a standalone sound word or as a modifier before 地 to describe the manner of the action. Note that this word also has other colloquial meanings (such as 'cigar shape' or 'a sly one') in some dialects, but its primary use in standard Mandarin is as a sound word.