I have always carried a gray card in my kit; almost never take it out.
I only shoot black&white now, so white balance is not a concern. I use incident readings for exposure.
You certainly can use a gray card for white balance. Just put it at a convenient place in the scene.
When used for exposure, the card should be angled midway between the light source and the camera.
That would also work for white balance, but the position is not critical.