Gibberish is typically only used by young children. Not appropriate here since you're likely not talking about a 3-5 year old.
Formal language is typically only used in a professional setting or with people you're not familiar with (i.e. not your family or friends)
Jargon is like slang, but it's used in a professional setting. Doctors use jargon when talking about diseases. Ex: A "JT" is a joint. This isn't slang you'd use unless you were in their workplace.
It's slang because we use slang when talking to someone we're familiar with and in a non-formal situation. Borrowing a bike from our brother is non-formal, and we're familiar with him.
