Can I safely connect my A6 GPRS module to this 5V 2A phone adapter
The standard output for USB is 5V but there are some such chargers that will output more for enhanced charging output on devices that support it.
In this case the adapter will default to 5V and will only upgrade to 9V if the device requests it.  The GPRS module will not request 9V and so the adapter will continue to provide only 5V which is what you want.