This is the very first time I’ve even heard about Gatehub or Bitmap. I wouldn’t recommend anyone to exchange their coins on a new website like those because they might not have the right amount of volume for you to even cash out.

Poloniex is alright, though there is a bunch of traders complaining about that website due to a number of reason. Look it up so you can be 100% sure.
Bittrex is the safest exchange out of those 4.