I am I required to bring a gift?

I second Edward. Card and wine. It doesn't matter if they owe you money, that is no excuse for being cheap. Either bring them something besides a card or do not attend the party.

If they are so darn wealthy, why do they owe you money? A card is fine.

I would not bring anything. After 50 years of marriage there is nothing you can contribute that she does not already have and at 50th anniversary he is supposed to buy her gold or something so whatever you buy will be put on a shelf, thrown away or re-gifted so why bother. If you don't bring anything and they say something then why would you care when you don't want anything to do with them anyway.

A bottle of wine, some flowers or a plant but never empty handed.

Show up with an anniversary card and a bill for the money they owe you.

I'd just take a card. Nothing more.

Just come! Don't have to bring anything. You are not close to them. Since they are wealthy, then why do they still owe you money.

You are not required to do anything. However you would be in poor taste to come empty handed. But if that's you, then be cheap.

Card and a bottle of decent wine ($15-20 range)