What Stan said (of course :-) ).
The grey card is likely to be "close enough" to spectrally neutral to be useful. The exact reflectance level is uncertain but also not crucially important - see below.
Many people sell this card set with a range of descriptions. Some say
- The Zeikos ZE-DGC Digital Grey Card Set features a digital grey color reference card, a pure white color reference card, a true black color reference card, a user's guide and a lanyard with a detachable clip. Get perfect exposures every time with this accessory!
One such example here along with a range of competing products.
While that does not tell you what it IS it does tell you that they think that they are trying to be "real".
Exposure: If you were using the grey card for exposure (which you aren't) it will ideally have a known percentage reflectance (12% or 12.5% or 13% or 14% or 18% or ... agh - the exact value is the subject of an ongoing many decades old holy battle some details here and the exact level does not matter as long as you know what results it gives you.
Color balance: For color balance the important aspect is that the card is spectrally neutral. If illuminated with "white" light from a perfect black body source it should reflect equal red, green and blue components. As the % grey in the previous paragraph varies the exact levels of R, G & B reflectance will vary, but they should be the same for each component. So, in your tests, if you were using a perfect white light source (we all should be so lucky :-) ), you would hope your grey card to return levels of (78, 78, 78) or (90, 90, 90) or (63, 63, 63) or ... . It is extremely likely that the people who planned the Zeikos grey card were trying to get spectrally neutral reflectance. It is almost certain that when making a ~= $10 3-card set with the grey "painted" on white plastic that they were not wholly successful in this attempt. It's also quite likely that the result will be "close enough" to be useful and that over time you will be able to make allowances for any slight bias that you may detect OR that you will find that any bias is small enough to not matter.
A number of user reviews of this specific card set are here. Of potential use. Users seem quite happy with the results.