Photography is approaching 200 years of age, first permanent image Joseph Niepce 1826. The digital era starts in 1975 when Steven Sasson at Kodak invented the first digital camera. In the early days, exactly how to expose and develop was hit and miss. It was Hurter & Driffield who conceived the science of Sensitometry. This is the science of how film and ...
Your example square is actually about 53%.
Upon reference image download exiftool shows that the ICC profile is ProPhoto RGB:
Kali:~ kelsolaar$ exiftool ***/rgbvalues.jpg
ExifTool Version Number : 10.80
File Name : rgbvalues.jpg
Directory : ***
File Size : 316 kB
File Modification ...
So many possibilities:
You are using the card incorrectly. (null) – When you set camera white balance, the card needs to be in the same lighting as the subject you are photographing. Place the card next to or over the subject when you set white balance.
You are using the software incorrectly. (EightBitTony) – I don't use Lightroom, so leave it to someone ...
A little history will help you understand the purpose of the gray card:
In the mid 1930's, Messrs Jones and Condit at the Kodak Laboratory determined that statistically, a typical sunlit scene integrated to a reflectance value of about 18%. About this time, the Western Electric Company brought to market the first light meter. Kodak Labs publish a ...