There was a long discussion of "собираться": http://masterrussian.net/f15/в-следующем-месяце-мы-собираемся-навестить-своих-друзей-19537/. To sum up what was discussed there, Russian "собираться что-либо делать" and English "to be going to do something" are not fully equivalent.

1. English "to be going" usually implies immediate future: "I am going to swim" is understood as "right now", "I am nearly in the pool".
Russian "собираться" does not specify when it will happen. "Я собираюсь искупаться" is more like "I'm thinking of having a swim", but if there is no sufficient context, it is not clear when the speaker will swim: in 5 minutes, tomorrow or next year.

2. Russian "собираться" does not even garantee the event will actually happen. The one who "собирается" can change his mind in some future. Unlike "to be going", the Russian "собираться" is thought as a process of "getting ready" or "making up one's mind", and it can be successful or not:
- Ты знаешь английский язык?
- Нет. Я собирался его выучить. Но так и не собрался.
- Do you know English?
- No. I was intended to learn it. But I failed to make up my mind.

3. Russian "собираться" can only be applicable to "conscious" actions. Normally, only people or animals (sometimes) can "собираться". In Russian, it is not possible to say
"Камень собирается упасть с горы" - it sounds funny as if the stone were a "thinking" object.
But in English, it is quite possible to say "The stone is going to fall from the mountain". Native speakers, please correct me if I am wrong.

There are a few exceptions of this principle. We do say "Дождь собирается" (It is going to rain) when we see some obvious signs in the sky.