Simultanous read and read/write on two serial connections
Adafruit_GPS.h (line 132-), which is included in Adafruit_GPS.cpp (line 13) refers in turn to the SoftwareSerial library on AVR platform and Arduino IDE version > 100.

#ifdef __AVR__
#if ARDUINO &gt;= 100
    SoftwareSerial *gpsSwSerial;
#else
    NewSoftSerial *gpsSwSerial;
#endif
#endif


This means it uses software serial on an Uno. It also means if your problem is indeed with Software Serial, that is the library to check for bugs / updates or commit a patch to.