Post an offer every week or every two weeks.
It’s a way to differentiate yourself and to get more people to your location. After all, if you know there’s traffic to your GBP listing, then why not make the most out of it and show people what you have?
This restaurant in Gilbert, AZ is doing it pretty well. Besides posts in general, they have offers as well.
It’s not hard at all. They have a simple “buy one - get one FREE” offer, and minus the fact that they didn’t put the date correctly, everything looks good.
They could also change the photo to show just the Shawarma Pita instead of 4 types of food. That would help a bit, but anyway... they are doing something most businesses don’t - so good for them!