Let me make a suggestion before we really derail into insanity.
A gray card is a tool, nothing more and nothing less. It is not a magic wand.
If you have a particular problem with your work, then look for a solution to that problem. And *IF* the solution looks like it might be a gray card, then by all means get a gray card and start learning to use it.
But do not get a solution and then start looking for a problem in your work to solve with that solution.
That isn't to say that a gray card may not be just the thing you need. But you haven't said why you think you need a gray card, and now all these old guys like me are making jokes about it.
But again, don't fall in the trap of seizing a solution then trying to find a problem to fit it. First try to identify what area you need/want to improve, then look for the tool to do that. And if coming to understand and utilize a gray card for critical exposure control is part of that solution, then jump all over the gray card.
But a gray card is not a magic wand. Don't dismiss it, but don't put too much value in it either.