I want to generate a constant monthly gain. Is it possible to do this by bitcoin trading? So, can I just do this by dealing with BTC trading?
Are there any people among us who can make a living by trading in BTC?
There is no easy way to make a living with bitcoin trading. You need a lot of experience in knowing where market will shift, which new coins are promising (ICO's can make a lot of money if predicted correctly). You need large capital and the mindset to handle losses; You can lose a significant money in a day because market or coin crashing, but you can't get emotional about it.
That being said, it is possible to get very decent monthly profits from trading. Just need a lot of experimenting, knowledge and capital. To start, try to learn as much as you can, deposit 0,05BTC for example to Poloniex, and practice. Set yourself rules to handle wins and losses, and make an investment plan.
WARNING: If you make investment, coin skyrockets, you make a ton of money,
DON'T start investing much bigger amounts; stay with your investment plan.
Since you used the word "standard"; I think in terms of bitcoins, there is no standard earnings you may get. Having to consider its value being volatile and changeable in mere seconds, you cannot actually say that you can have that constant amount you may earn. Some trades may win, but some may not, so basing your living in it would be a big risk.