GSM antenna inside metalic enclosure
As you say, the enclosure will block radio waves emitted inside of it.

People often think that a shield should block all radio waves.  Technically it does, but in practice it doesn't because the shield itself re-radiates some of the power.  You may be able to use this to your advantage.

If one of the dimensions of the box is a multiple of &frac12; wavelengths, then use the box as your antenna.  Connect the two antenna feed lines to opposite points of the box on the inside.  Unless you have sophisticated antenna modeling software, you'll need to experiment with a field strength analyzer to see what connection points work best.