[발췌: In casual speech] Have got to V, 've gotta V, Gotta V,

출처 1: http://www.learnersdictionary.com/qa/what-does-gotta-mean-and-how-is-it-used

Gotta is a way of writing got to that shows how these words are pronounced in the expression have got to in casual speech: have gotta. Have gotta is an informal way of saying that something is necessary or must be true.

• I've gotta leave in 15 minutes. (= it is necessary that I leave in 15 minutes.)
• You've gotta be kidding. (= it must be true that you are kidding.)
• We've gotta learn to get along better. (= it is necessary that we learn to get along better.)

In very informal language, gotta is sometimes written and spoken without have, as in these examples, which have the same meaning as the sentences above:

• I gotta leave in 15 minutes.
• You gotta be kidding.
• We gotta learn to get along better.

With or without have, gotta is very casual, and should only be used in informal conversation or when writing a representation of informal conversation.


출처 2: https://english.stackexchange.com/questions/229693/i-gotta-go-or-ive-gotta-go

( ... ) "I gotta go" is merely a phonetic representation of the relaxed pronunciation of "I've gotta go" and the 'v' sound simply being missed out ( ... ).

It is similar to the phonetic writing of the word "have" as the word "of" as in the example "I would have" or "I would've" being written as "I would of" simple because it sounds like it ( ... ).

( ... ... ) 'I gotta go' [even] descended into 'I gorra go', and still just 'gorra go' ( ... ).




출처 3: 

CF. Pronunciations of “ ’d ”