They are not interchangeable. To be "about to" do something is a pretty immediate action, moments away from happening. If you are "going to" do something, it could be in a moment, or in a matter of days or weeks. "Going to" just shows your intention to do the thing, "about to" shows both the intention and the time reference.

Thank you Henry for your detailed explanation.

No, they're not interchangeable. "Be about to" means you're on the point of doing something or on the cusp of carrying out an action. And it's not necessarily absolutely certain. Often you say "I was about to do something, when something else happened". In this case it's an interrupted action. "Be going to" is similar in the past tense. "I was going to do something, but something else happened". I.e. Your plan was interrupted. Alternatively, if it's in the present tense, it just means you're discussing your plan. E.g. "Tomorrow I'm going to go to the shop". Hope this helps!