I'd have a quick check about the GSM board's requirements... I thought most of them had a 3.3v regulator on-board so you may find you can run them directly off 3.7v, which makes you life a lot easier.
If so that makes your life a lot easier. Just get either some phone batteries and connect them in parallel (Adafruit sell a charger board but it only handles a single cell's worth of voltage)
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