There are several gray scale charts available: 3,5,7,9,11,13 steps, linear/logarithmic, transparent/reflective, different sizes... So it seems to be difficult to choose the appropriate chart for your needs. But when following SOME RULES it gets much easier to find the right chart.
CONTRAST First thing is to decide if you need normal contrast just to check the gamma settings of your camera or if you need high contrast to check the dynamic range (contrast the camera can see and transfer into a picture).
REFLECTIVE OR TRANSPARENT For normal contrast – up to 200:1 – the reflective charts are suitable.
For high contrast – 1000:1 up to 1000000:1 – a transparent chart is necessary. Here you will need a proper illumination from the back side like an integrating sphere.
GAMMA / LINEAR For checking the gamma setting of a camera gamma a chart with typically gamma 0.45 is used. The linear charts can be used to check cameras where the gamma is switched off or where no gamma is needed (like industrial machine vision for example).
CHART SIZE The size depends on the camera and the used lens, for wideangle lenses the bigger ones are easier to use (you do not have to be so close to the chart).