I have seen many sites like this one. Most of them proved to be a pure scam. Firstly they increase your balance like they promised and when you try to withdraw your funds they will show minimum requirement to withdraw you balance. Beware of those sites. Better try trading and buy some shitcoins.
Gainbitcoin is no different, they are a scam!
I was there myself too, luckily i have my investment back and due to the increase of bitcoin price i am ok.
Now they claim to have " big problems" due to heavy congestion with payout, but offcourse this is a lie. They implemented some sort of congestion meter on their website which states that as long as the congetions is 100% it's impossible to payout. They only play mindgames to keep low-profile about this, but their intention is to avoid withdrawals.
A few weeks ago, they also stopped replying support requests and now they have made it impossible to raise one. I will not even try to withdraw, or raise tickets anymore, i know enough.
All the signs are there, the ship is going down.