The "0" in the dtostrf() call is the minimum width of the output field, and while poor programming practice, works fine. It is not clear to me what the function author(s) intended with that parameter.
@beryindo: make sure you can read the GPS and get sensible values on the serial monitor, before sending them to the phone company. Start with the neoGPS provided examples AFTER reading the documentation. From your reading, you will learn, for example, that
IMPORTANT: gps.fix() IS ONLY VALID WHEN:
gps.decode() just returned DECODE_COMPLETED, or
gps.is_safe()