Power supply for A6/SIM900 GSM module
It all depends on your power supply. The module seems to use about 1A (from the source you linked). If you are powering the Pi from a 2A microUSB connection as recommended, and you don't have any other peripherals (e.g. a Wi-Fi dongle), then yes, it is okay to power it from the 5V pin on the GPIO header. What another user posted about was the generic GPIO pins. These provide ~20 mA @ 3.3V, and can be switched by software. You would not use these, but the dedicated 5V port on the module.