Not quite I think. Some camera require gray card for WB adjusting, or can use gray card for WB also.
But I think you are right, i saw few discussion suggesting 18% gray card. But 18%, any advise?
Based on my understanding, the gray card should not have any color tone in it. So, if you made it youself at home with a color printer, you will need to calibrate your printer properly to make sure that the gray is gray, not with some color tone. Having said that, I wander if you just print it with a black and white lazer printer? Hope some bros out there can advise.