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?
No. It is not possible to have fixed monthly income/profit with trading. Because the prices are changing continuously so there's no assurance that you can earn fixed. There are people who are making trading as their main source but it is better to have some other way.
is the vexatious sentence in the OP. I quite agree with Tabas that trading isn't a pre-determined involvement where you already know what should come out of it. No one can conveniently tell you exactly how much they can make in a month or in a trade until the trade is executed. It is a give or take thing. To that I say, No! No one makes constant gain trading. Expect loss too.
Theres no really constant earnings on daily basis on making tradings and as you said we wont really know on how much we would able to make on such trade and we wont still know if we would make profits or make loses on that one. Its really hard to make profits if you dont really know how to trade on Bitcoin knowing that its price is really unstable. It do go on random ways which is really a tough job.