You will need to calculate this using the difference in the altitude values from two data points over a given time.
I do this for my Flight Instruments data field -- https://apps.garmin.com/en-CA/apps/87748510-5c7f-48c3-a2bc-2b09e006f0f4 -- though I have some more complex code to make the calculation less "jittery" and thus make the end result a bit more like the actual trend instrument in an aircraft.
