softwareserialgsmerror.ino

#include <SoftwareSerial.h>

SoftwareSerial gsm(5, 4);

void setup() {
  Serial.begin(115200);
  gsm.begin(1200);

  Serial.println("\GSM test started");
}

void loop() {
  char cmd = 'p';
  int brate = 115200;
  while (gsm.available() > 0) {
    Serial.write(gsm.read());
  }
  while (Serial.available() > 0) {
    if (Serial.peek() == 'b')
    {
      Serial.println(Serial.read());
      cmd = Serial.read();
      switch (cmd) {
        case '1': brate = 115200; break;
        case '9': brate = 9600; break;
        case '0': brate = 1200; break;
        default: Serial.println("Error command");
      }
      gsm.begin(brate);
      Serial.println();
      Serial.println("Baudrate change to"+String(brate));
    }
    gsm.write(Serial.read());
  }

}

//Below are tested results

tested result.txt

GSM test started
⸮⸮⸮98
Baudrate change to-15872
⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮⸮98
Baudrate change to9600
⸮⸮
+CREG: 1,"1F57","9D3C"

+CGREG⸮