'I'm gonna bath you' means you're going to give a bath to the baby. 2nd sentence makes no sense. DON'T use 'gonna' in writing it marks you as a 'beginner' who's relied on sub-titles/US tv shows, etc. Native speakers do not write this if they have more than an elementary education. Using this seems childish or someone who has a very poor education. I'm not insulting you, I just want to let you know how this usage is perceived by most native speakers. I *think* learners mistakenly think this sort of writing is normal. It's not !

1 - I completely agree with Greg about "gonna." It looks wrong every time I see it on these forums. For the most part, native writers do not write the word "gonna." Most of the time, we do not say "gonna," we say "going to." We say it really fast and we slur the words together and it might sound like "gonna," but if you asked someone to write down what they said - they would most likely write "going to." Just remove "gonna" from your vocabulary. It does not make you sound more like a native speaker. Definitely do not write it. 2 - "I'm going to give you a bath." You could also say "I'm going to bathe you." But "bathe" has a slightly formal tone and it isn't what I would say to a baby. 3 - "I'm going to breed you" just sounds disgusting. I am not sure what you mean, but it almost certainly isn't that. (If I wanted my dog to have puppies, I might say "I want to breed her with the neighbors dog, so they can have puppies.")

‘I’m gonna’ is American slang for ‘I am going to’. It is fine to use in most conversations, except very formal occasions. ‘I want to breed you.’ doesn’t make sense here. To breed is used when animals produce offspring. ‘We hope our pigs will breed soon, so we will have piglets to sell.’

I'm gonna' give you a bath. I don't understand the second sentence.

No, neither is correct. I am going to bathe you means I am going to give you a bath. I’m not sure what you are trying to say in the second sentence, sorry.