It's not a very common word in today's English. In Britain we do often refer to it as prison or jail, and most of the time it is spelt the same way as it is spoken. Gaol is more old English than the English that we commonly speak today. The pronunciation is much the same as jail though. In conversation the spelling of a word isn't quite as important as the pronunciation.
on Nov 11, 2017, they gave the translation as "but one prison is a prison" - but why not "but one prison is one prison" -- if one's uptight about having ANY prisons in one's city you could complain about having one already, with the risks / stigma attached, if they proposed having two -- like some US cities -- you could complain
What about the meaning that not all prisons are jails, but all jails are prisons. Jail is short term but prison is long term.
For the difference between jail and prison, see: http://answers.yahoo.com/question/index?qid=20080101131101AA2tpCy
So I took the meaning to be, "If you are in jail (short term), you are still in prison (long term); that is, either way, you are still in jail. I think my answer should be accepted.
It's not exactly synonymous... A jail is technically a holding place for pre-trial detention where as a prison is usually a place where longer-term inmates serving actual sentences are housed.
I am not sure if this difference is formalized in the English language or is just a convention, though.