The word 'gotta' is not a real word... it is the spelling of the sound of the two words 'got to'. When people say the two quickly, it sounds like 'gotta'. 'gotta' is the same as 'wanna' (want to). These are not real words, so I suggest that you do not write these words and try to pronounce the words separately and clearly... got to and want to, not gotta and wanna.

Gotta is an abbreviation also informal. Gotta is used to say got to or have to. Namely You can say I gotta go! Also I got to go. I have to go. So gotta is used in informal english. Maybe you say why people use gotta instead of got to, have to because it is shorter so easy to say it also maybe cool:) and umn maybe it can be lazy of people..

Richard is absolutely right. The meaning is the same as "have to" or "must".ex: I got to go now.