i thing buy some Gh/s from Minergate Cloud Mining is good idea?
It's your choice. But i would not do it.
People have said that minergate is an illegitimate company, and with so many complaints with changelly(its sister company), it's easy to see why they have a bad reputation. And also, the hash rate available for purchase never runs out. If they are indeed mining, don't you think that they would at least put a cap on the amount of hashing power available for purchase?
Even if they are actually mining and not running a ponzi or whatever, the returns you'll see will be extremely small if at all.