Does my creature get stunned after killing Pip Pip with a Red Planet Ray Gun?
The active Player chooses

Do you want your creature to be stunned? If there are multiple abilities that trigger after the reap, the active player chooses the order to resolve them in. So, if there was some reason you wanted your reaping creature to be stunned, you could resolve Pip Pip first, then the Ray gun. If you didn't want to be stunned, you could resolve the Ray Gun first, killing Pip Pip before it's ability could be applied.