I think for me you could see Mane suited exactly what Klopp was trying to do. He was dynamic, hard working, could score and assist, was perfectly suited and already adapted to the Premier League, and the most important thing is that he bought into the press from the front, which for me was the basis Klopp built his success from, as all 3 forwards bought into this.
I just don't think Gakpo is that type of player, certainly in the games I've seen him for PSV against British opposition in Europe (x2 vs Leicester, x2 vs Rangers and x2 vs Arsenal). So while I think he does have some very good qualities, (his ball striking being 1 of them), I must admit I'm quite happy to see Klopp having to deviate from that elite pressing machine that made them so horrible to play against.