[needacig]

Why not try to carefully stick up for this non-present colleague? As long as you don't make the person who is gossiping feel attacked or belittled, this could effectively discourage the behavior. The trick is to convince the group that you are above petty complaining about others. People who are positive, smiling, and upbeat tend to be more likable than those who are negative and complain, like this gossiper seems to be. Maybe just gently smile and point out the good sides of this non-present person. If the gossiper says something negative about this person, try to downplay it by saying something like "Oh, I've had that happen to me too".

If you really want to leave the lunch, you could have "an important lunch meeting" conflict with this one (ask some other coworker to have lunch with you to ostensibly talk about work), or claim that you have an important deadline to meet and must work through lunch for several weeks. After a while, they will forget to ask you.

[needacig]

The trick to not getting into an argument with this gossiper is to always seem like you have the utmost sympathy their point of view, but you can also defend the other person by playing as if you are extremely sympathetic to them too. You will come off as just a really nice person, and most people will like you for it.