I have read that usually a gsm module requires a power supply capable to provide up to 2A. But how can 3g modems sticks work properly when they are powered through usb port which provides only up to 1A(?)
Here is a very related question. This modem has 25% duty cycle, so 2A peak translates into 500 mA average. And yes, the voltage drops noticeably when RF transmits, so certain care must be taken to bypass this drop. Other people indicate the amount low-ESR ceramic caps of the order of 2,000 uF, some recommendations go up to 12,000 uF.
Obviously, for the bus-powered USB dongle to meet inrush current requirements, a special gradually-working power management is needed.