Essentially, gubbing is the act of having your account limited by a bookmaker. You'll get an email from the bookmaker stating that you have in fact been gubbed. Their email won't mention this term, but in amongst all of their jargon the same message will be passed across.
Of course, if you really need your money you are able to withdraw it whenever you desire. However, if you don't urgently need your cash you should consider leaving the money sitting around in your account to use on future offers.