SoftwareSerial is an architecture-specific library. The version you found at C:\Program Files (x86)\Arduino\hardware\arduino\avr\libraries is written specifically for the AVR architecture (e.g. Uno, Mega, Nano, Leonardo) and will not work with your MKRGSM1400, which is SAMD architecture. You will probably find some relevant information if you do a search for arduino zero softwareserial since the Zero is also SAMD and has been around much longer than the MKRGWM1400.