An adverbial mimetic meaning 'in large quantities' or 'making/receiving a lot' (often used for money or profits).
あの会社は新商品でがっぽがっぽ利益を上げている。
That company is making a lot of profit from the new product.
宝くじでがっぽがっぽ儲かったわけではないが、少し助かった。
It's not like I made a fortune from the lottery, but it helped a bit.
彼は一度成功するとがっぽがっぽ稼ごうとするタイプだ。
Once he succeeds, he's the type to try to rake in a lot of money.