The issue of taxi cab drivers who stand by the hotels to pick up customers, and while standing there they give some money to the guards or gatekeepers to pick a better customer, so, such doings is ok or not?

The employee of the hotel in this case the gatekeeper is not responsible for choosing a particular taxi for customers or vise-versa. Hence if some particular customer is suggested by one of the two parties then to give money to get that customer is permissible.
Original Source Link (from Way Back Machine archive)