I'm not sure of the exact origin. However, there are people that consider it to come from North American speakers. The North American teachers I've worked with, used it far more than teachers from other countries. They are VERY lazy and sloppy contracted forms of, 'want to', and 'going to'. It is also considered slang. DO NOT use it in an IELTS test. It's also not a good idea to use it in writing, unless you are quoting somebody, "I'm gonna leave". I have heard people say it in England too. Personally, I don't like it. I teach my students, 'going to', and 'want to'. If they are teenagers or adults, then I make them aware of 'gonna', and 'wanna'. Enjoy your learning.

even here in our country, these 'informal' words are commonly used. :)

It's been my experience that the British also use "gonna" and "wanna" in normal everyday speech, as do Australians. I don't think anybody necessarily copied anybody else, but the tendency to use less effort in speech is almost universal.

I'd say it is, but I wouldn't use it when speaking to your boss.

I use it as some kind of short form for 'want to' as wanna and 'going to' as gonna it is a kind of slang to me and it is not compulsory to learn it as part of learning english. :)