When your client is the government, and you are managing a mixed team of your own with government employees, when is it appropriate to provide non-monetary rewards (such as dinners or gift certificates)?
- Ask your contracting officer for any compliance guidelines or check online guidelines published by the relevant compliance agency e.g. FAR for US Gov't DoD.
- Whenever the group reaches a particular milestone (e.g. kick-off or close a project), when they need a group team-building moment or for a recognized national holiday it should be appropriated to take everyone out to dinner or bring in pizza, etc.
Where you may create the appearance of impropriety is when you do things for specific individuals rather than the group.
I am not sure if this question will be closed or not, but if is not, I think there is something you can do. From your question I understand that you are running the PM activities for a government project. Your team members are government employees, so gifts "might" be mis-interpret.
What I will do is, create a reward plan before giving any rewards. Usually if you have an objective plan before hand, you will show it to your sponsor (customer in this case), and get them on-board. If they say, is ok to give a dinner gift card when x,y,z happens, then you are set. If not, they will tell you what is allowable.
That's my two cents.
To avoid running afoul of anti-bribery laws, regulations and policies, you'll have to check with your contracting officer for guidelines. Each agency will have different policies, so there is no answer that can apply to all government workers. The laws will also be different for federal, state, county and city governments.