The earliest temperature gauge is generally attributed to Galileo Galilei around the year 1593. He created a device called a thermoscope, which was a sealed glass tube filled with water and a bulb at the bottom. As the temperature changed, the air inside the bulb expanded or contracted, causing the water level in the tube to rise or fall.

However, it's important to note:

* Galileo's thermoscope was not a true thermometer: It lacked a calibrated scale and couldn't provide accurate temperature readings.

* Other inventors around the same time were also working on similar devices: Santorio Santorio developed a clinical thermometer for measuring body temperature, and Cornelis Drebbel experimented with sealed air-filled tubes.

Therefore, while Galileo is credited with the earliest temperature gauge, it was a rudimentary device and the true development of the thermometer with a calibrated scale came later. The first reliable thermometer with a standardized scale was invented by Daniel Fahrenheit in the early 18th century.