“Is paying a large tip to secure a better table in a restaurant the same as offering a bribe in business to secure a contract?” I think if we look at the other factors coming into consideration that they are different which leads us to conclude that these two actions are not the same. However looking at the moral philosophies behind each one, there still remains the question of is it ethical.
The Merriam-Webster online dictionary defines a gratuity as "something given voluntarily or beyond obligation usually for some service". In most cases (especially in a restaurant scenario), it is given for services rendered after the services have been received. In some cases, a tip is given to encourage a certain behavior and engender favor towards you. Think about this for a minute: If you were a server in a restaurant and you have two tables to attend to. One of them is a couple who regularly dines there and leaves large tips while the other is a grumpy man who only complains or says nothing and is a stingy tipper. Which one would you pay more attention to? While you might do your job well and bring them both their food and drinks, would you be more inclined to make small talk with one over the other? Which one would you offer a discount coupon to? While the customers have not been bribing you, the gratuity has affected your goodwill and changed your attitude towards them.
One good rule (though not the only guidepost) in ethics is to ask yourself how would you feel if your employer knew about the actions. Most unethical behavior is dishonest and illegal but I daresay that in the scenario given above, your employee might even encourage the sort of behavior that generates repeated sales.
When an employee has a personal interest in a transaction that is not disclosed to his employer, then it often results in a conflict of interests and fraud. Sometimes an outside party (such as a competitor) might offer a gratuity...