I haven't tackled this reqt but I have produced AFM landing charts with different approaches specified so just a thought.
An instrument displaying gradient is not required.
You need an instrument providing information.
From which gradient is displayed - debate how - and the gradient is an operating limitation.
VSI and ASI provide information.
Provide AFM chart with the gradient limitations shown as ROD for the appliable approach speed. Toss in altitude and temperature in the chart.
Fly the aeroplane at the specified approach speed. Set descent profile within ROD parameters from the chart.
I could point at that chart and state that the pilot is provided with instruments which provide information enabling the chart to display the approach gradient.
If it was me, I'd be looking at AFMs to see what others may have done.