There are a few ways but I would Make some instance Variables for your Player that are something like
and set all defaults to 0
Next make an Event somewhere near the top of your Sheet..-> Every Tick-> Set Value"Player_Fall_Distance" to "Player_Final_Y" - "Player_Start_Y"
Next make a New Event under the one you just did above
but make the Conditions..-> Player -> Is Falling & Do once only
& the Actions something like...-> Set instance Value "Player_Start_Y" to "Player.Y" & reset "Player_Final_Y"
This makes sure that on the beginning of each fall that the Final Y is reset not that it really needs it coz its updated..but yeah doesnt hurt to make certain
and then another Event Condition that goes Player -> Is on Floor & Do once only
and the Actions for this one....-> Set "Player_Final_Y" to "Player.Y" again..
and the only thing you need then is Another Event that Checks The result of "Player_Fall_Distance" to the Distance value you want
and make an Action the Kills the player or drops their health by the amount you wish...
And one last thing....
You would need to reset all of the Variables I think...but maybe not
I havent tried this yet...let me know if it doesnt work and I'll have more of tinker...lol
Hope I didnt miss anything