Gauge mode is what you use if you want to use it as a bottom timer/depth gauge only. You can continue to use it in that mode for subsequent dives. It assumes you are using tables or a PC print out dive plan for your nitrogen or oxygen loading. Usually this mode is used for technical diving where various mixes of gasses are used, and/or tri-mix which the computer cannot calculate nitrogen loading for.
Normal mode is what you want to use if you want it to use the nitrogen tracking features of the computer, and use the computer for subsequent dive planning based on your nitrogen loading.
The reason why you can't switch it out of gauge mode after you made a dive is that the computer did not track your nitrogen loading for the first dive, so it would be impossible for it to calculate planning for any subsequent dives. If theoretically you could put it in normal mode, the planning data would not be valid because you have residual nitrogen in your body and the computer has no idea what it is because you didn't dive in normal mode your first time.
So this is not really a problem, it is the intended use of the computer you can only use it in gauge mode if you put it in gauge mode. The way to solve any future issue is to NOT set it to gauge mode if you really want to use it in normal mode.
