Is it Possible to make a GAMEPASS from a KEYDOWN function?

Yes, of course, but you'll need a RemoteFunction to return whether you own the game pass from a LocalScript. The jump Bindable should also be disabled. You can also view this for some extra knowledge.

LocalScript in StarterCharacterScripts

local me = game.Players.LocalPlayer
local char = me.Character or me.CharacterAdded:Wait()
local humanoid = char:WaitForChild("Humanoid")
local RemoteFunction = script:WaitForChild('RemoteFunction')
local UserInputService = game:GetService('UserInputService')
local canJumpAndRun = RemoteFunction:InvokeServer()
-- Invokes the remote function that return whether you own the game pass
    -- only checks when they load though, so if they buy it after, they'll need to reset. 

local ContextActionService = game:GetService('ContextActionService')
ContextActionService:UnbindAction("jumpAction")


local maximizedWalkSpeed = 30
local defaultWalkSpeed = 16

UserInputService.InputBegan:Connect(function(input)
    -- Something was pressed
    if input.KeyCode == Enum.KeyCode.Space then 
        -- Will need a loop to check if we're holding to keep jumping
        humanoid.Jump = (canJumpAndRun and true or false)
    elseif input.KeyCode == Enum.KeyCode.LeftShift then 
        humanoid.WalkSpeed = (canJumpAndRun and maximizedWalkSpeed or defaultWalkSpeed)
    end
end)

UserInputService.InputEnded:Connect(function(input)
    -- Key up
    if input.KeyCode == Enum.KeyCode.LeftShift then 
        humanoid.WalkSpeed = defaultWalkSpeed
    end
end)

In a Script inside of the LocalScript

local RemoteFunction = Instance.new("RemoteFunction", script.Parent)
local GamePassService = game:GetService('GamePassService')
local GamePassId = 100000

function RemoteFunction.OnServerInvoke(player)
    return GamePassService:PlayerHasPass(player, GamePassId)
end