Does gray card a necessary tool in photography? Do you have any gray card with you? I do. I have Mennon Gray Cards. However, I did not use it much since I bought it. 
Mennon Gray Cards
The Mennon Gray Cards package that I bought consist 2 gray cards. They are 8×6″ and 10×8″ sizes. I did use 8×6″ gray card once in a while. But I did not use 10×8″ at all because it is too big to carry along.
8×6″ and 10×8″ Mennon Gray Cards
So what does gray card do? It will correct your photo white balance. Just look at the photo below.
Original photo
White balance corrected with gray card
As you can see, the first photo is warmer than the second one. The warm lighting is causing the photo to look warm. However, by using gray card, we can correct the white balance of the photo. 
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how much u get the grey card?
might wanna get one too ..
I got it for RM51.
how to use the gray card?
hold it beside the camera to block out the light??
Hmm… Hard to explain it here. But definitely not to use it to block the light.
Oh dear, I never use any grey (or white) cards to correct white balance. I find them too much of a hassle (imagine shooting grey cards every time the light temperature changes…).
I find shooting in RAW is the safest to get correct WB because it can be tweaked later in post processing without any problems. Shooting in JPG allows tweaking too, but not as much lee way.
Basically I just make sure I choose the closest WB setting in the camera, then correct them during post processing.
On a related note, I find correcting WB problematic when there are 2 types of lighting with vastly different temperature (say tungsten+daylight or flast+tungsten).
Ha…Ha… My gray cards stay at store room most of the time. Just want to test it so bought it.
I shoot only RAW. Unless really not space in my memory card. Anyway, I do post processing all the time.
Do you use the camera’s light correction systems too? Or does using the gray card mean you need to keep the WB setting no matter despite different lighting (outdoor, indoor, tungsten, etc)
I use auto mode all the time since I shoot in RAW. I can change it later with Canon RAW converter.