MG2639 & Arduino Uno not GPS-ing?
Try sending a command that sets the GPS baud rate to something a little lower:

altss.write( F("$PMTK251,38400*27\r\n") );
altss.end();
altss.begin( 38400 ); // change to match above command


Are you sure you're having trouble reading, because there are many other things you may be doing in your sketch that cause input buffer overflow.  Please post your code and some sample output for us to examine.  It may be failing for a different reason.

For example, you may just be losing data because you're trying to do too many things in loop().  Please take a look here.

To solve this kind of problem, I have posted a modified version of AltSoftSerial on github, here, that let you process GPS data in the background (i.e., during interrupts).  When coupled with NeoGPS, it can save you quite a bit of program space, RAM and execution time.  There is an interrupt-driven example here.  Just change which include is uncommented on lines 30-31:

#else  
  // Only one serial port is available, uncomment one of the following:
  #include &lt;NeoICSerial.h&gt;
  //#include &lt;NeoSWSerial.h&gt;
#endif
#include "GPSport.h"


...and the baud rate below, on line 140:

  // Start the UART for the GPS device
  gps_port.attachInterrupt( GPSisr );
  gps_port.begin( 115200 );
}