Gaussmeter is a scientific instrument that is used to measure the magnetic field strength and direction, it is really a handy tool for every hobbyist and quite a fun to tinker with.
The key component is the MLX90393 this is 3-Axis magnetic field sensor and is quite capable of what it does.
I have also made a complete video tutorial for the same project and is the following:
Make the connection from module to Arduino for I2C, that is SDA to A4, SCL to A5, GND to GND, and 3.3V to 3V3.
Connect the Arduino to the PC and open the Arduino IDE.
Go to Sketch, Include libraries, and then manage libraries.
Type in the search MLX90393 you will see the library by Aafruit, Download it.
Go to files examples then Adafruit MLX90393, open the basic demo sketch.
Now as the sensor is 3-axis one, but we are only interested in the axis that is perpendicular to the plane, that according to its datasheet is Z, so in the sketch, I would recommend commenting the Serial print line of x and y-axis.
Upload the sketch, after the upload completes open the serial monitor at 9600 baud rate.
You should see the magnetic field on the axis you did not comment out, to measure the magnetic field strength and direction in teslas bring magnet closer and perpendicular to the sensor on the breakout board.