When is a gift become a bribe?

It has to depend both on the giver and the receiver’s intention. It can be of any value and it can be either in cash or in kind. If I am giving something to somebody without any expectations of a return favour then it is a gift and when I am giving money to somebody coz I am pleased with the way I’ve been served or because the other person has done a very good job, then that is tip. Bribing is only when there is any expectation of any return favour.

It would depend on the giver's intention, regardless of its value. If it could affect a certain outcome/decision then I think its a bribe. I think it's different from a tip in a way that you give tip because you are satisfied of the services you have received, not like a bribe wherein you give it because you want something you can't get without influencing others.

When something unbecoming of a person's position or responsibility is expected out of it either directly or indirectly.