Yes, only suitable for informal use. Also usable only in North America.

I would generally only use those expressions in writing if I were writing dialog. Otherwise I would write "going to." "Kind of" and "sort of" are somewhat ambiguous. They are not terms that will often be found in advanced writing. "He was kind of angry." = "He was rather irritated. "or "Although he was smiling, everyone could see that he was beginning to lose his temper." "She is sorta tall." = "She is taller than most of her friends." or "She is tall enough to pass for a 14-year-old, even though she is only 12." I'm available for lessons! : )

"Are expressions like "Kinda", "sorta", "gonna" too informal" They are informal. I would not use them when writing a report. However, if writing dialog (as mentioned by Jordan) between friends, I might use them depending on the situation and where they were from. They are the made up words by combining two words into one so as not to have to enunciate (use the muscles in your mouth to differentiate sounds). Kinda= kind of. I kinda like this spot. I kind of like this spot. (Still informal). Someone might say, "I like this kind of chocolate," but I don't think they would say, "I like this kinda chocolate." sorta= sort of. I sorta like this spot. I sort of like this spot. (same meaning as kinda and also informal even when separated into two words). Someone might also say, "He is sorta/kinda short." This would mean while they are not necessarily short, they are somewhat short. Some might consider the person short and some might not. Maybe he is on the shorter end of average. gonna= going to. I'm gonna go home now. I am going to go home now. (When made into two words, this is at least acceptable in other settings as well.). "I'm gonna get you!" is probably more commonly used when playing around and said with a laugh, than "I'm going to get you!" which is probably more often said in a mean tone of voice.

Yes, they are!

Yes these are colloquial expressions.