If you've got lots of money you can invest in a widget system, which can be bought with an integral pressure gauge, but it is very expensive. Otherwise - assuming that you have a wide 4" dia capped keg - Leyland Homebrew offer a fitted pressure gauge to a new keg for £7.95 (code 0812b) - If you ask them nicely perhaps they will supply one that you can fit yourself, assuming you can drill a hole in the cap?. They can also supply an injector valve for use with the standard S30 cylinder if your keg does not have one fitted. If you look at the picture of the pressure gauge on their website you will see that it is actually fitted to a Hambleton Bard cap with injector and relief valves also fitted.
http://www.leylandhomebrew.com/cat46_1.htm
As to how you are to release pressure, I can only suggest you carefully and gently insert a thin piece of plastic (a toothpick for example) between the plastic sleeve and body of your pressure relief valve near the outlet hole, thus relieving the pressure over the hole allowing gas to escape - a far from ideal solution I know but the only one I can think of.
If your keg has the narrow 2" dia cap, I don't know of any way you could fit a gauge and injector/relief valve, there just isn't room. Unless you invest in the expensive widget system all you could do is replace the fitted relief valve with a combined injection and relief valve to enable forced carbonation.
Finally and perhaps most importantly I would like to reiterate the point made by David 42 above -
Do not remove the currently fitted 10 psi pressure relief valve unless you replace it with a similar one (ie combined injection & relief valve). Kegs are fitted with pressure relief valves with good reason - a split keg and 40 pints of your finest all over the floor is not a good start to your brewing hobby!.
Good luck & happy brewing - Buster, Northampton, England.