Other than the messaging platforms that you said, we can also post the problem directly to the public chat of a gambling sites so that other players are going to get aware of the issue. Some gambling sites also have their own communities or forums, you can also create a topic there.
From those who has messaging option for example (telegram chat/Facebook chat etc) can be manipulated from alt account/accounts and also they might be able to delete any negative comment/views from telegram group and they can do the same on other messaging chat. I think bitcointalk forum is the best one from where we can get enough information about a gambling site by asking on the forum if that is not listed on the forum and if listed then there is already enough information.