Someone who plays with dice is only posul for eidus if he does it for a living, from what I was taught in yeshivah. Maybe that is because he is willing to risk anything on teh turn of the dice, so he does not ahve the proper mindset for the chashivus of honest eidus.
The funny thing is when I saw the title of this thread, “Kvitlach, Muttar or Assur.” I actually thought it was about the permissibility of putting little messages into the cracks in the Kotel.