What's the meaning of انگار here: It maybe means "apparently" in these cases, when you strongly guess something. What does غش غش mean here: When you laugh loudly. "ghash ghash" is not a certain word, it is a sign of a sound when you are laughing very loudly. What does راست شكم mean here: This term almost uses in an informal spoken and it is a little bit funny term. That is not very common. But it means when you don't pay attention to do anything except what you want to do. Sometimes you have to do something but you don't do it and ignore that and go.

اِنگار = as if غَش غَش خندیدن = to laugh hysterically راستِ شکم را گرفتن و رفتن = to go your own way, often indifferent to what goes on around you

Ashkan's answers are complete but about the last phrase, it means when you walk straight along without looking back and don't pay attention to your environment because it's not important for you and we have another phrase which is so close to this one. we say : راست دماغش رو گرفت و رفت it has the same meaning.