I think taking your work colleague out for a meal is a nice thought. A gift certificate is a nice thought too.

If it was me (and I am quite different), I would think it more than acceptable to receive the gift certificate without being shouted a meal. After all, if he wanted to share a meal with you to celebrate, you would all be invited to the wedding. (I do realise that wedding budgets don't always stretch that far.) What's wrong with the certificate, dedicated to a goal or not ($10 per person), and giving him a special morning tea at work?

After all, it's the thought behind the gesture that counts.