Gold buying is the root of the problem. GDKP indirectly fuels gold buying.
Gold buying is supported by Bots, ruining the gaming environment for legitimate players.
Due to the excessive gold generated by bots which is then bought, the economy inflates effectively making legitimate gold earning less rewarding - if a daily quest returns 10g and a flask costs 10g, it takes 1 daily to pay for a flask - if the flask costs 50g, now need to do 5 daily quests to pay for the flask. If nobody can afford the 50g for a flask, the price drops - market forces.
Without gold buyers, GDKP would be significantly less lucrative. Active players able to earn 10k a week generally would have gear already from their guild runs - since they are a very active player. GDKP works for inactive players/people with jobs who can then get the same gear by buying gold and spending it in GDKP runs. Therefore, GDKP fuels negative behaviour (bots and gold buying), hence the negativity towards them.