Use UserOwnsGamePassAsync to check if the player has a pass:

local gamepassId= YOUR GAMEPASS's ID

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function()
        local hasGamepass = game:GetService("MarketplaceService"):UserOwnsGamePassAsync(player.UserId, gamepassId) 
        if hasGamepass then
            plr.Character:FindFirstChild("Humanoid").WalkSpeed = 200
        end
    end)
end)

Preventing speed hackers:

Keep in mind that hackers can change their characters physics which means they can change their speed. You should have some sort of check to see if they DON'T own the gamepass and they're faster than normal.

The script would look something like this:

local gamepassId = YOUR GAMEPASS's ID

local previousPositions = {}
local MAX_SPEED = 25  

function checkMovement(player)
    local character = player.Character

    if character then
        local hasGamepass = game:GetService("MarketplaceService"):UserOwnsGamePassAsync(player.UserId, gamepassId) 
        local previousPosition = previousPositions[player.Name]

        if not hasGamepass and previousPosition then
            local distanceTraveled = (Vector3.new(previousPosition.X,0,previousPosition.Z) - Vector3.new(character.HumanoidRootPart.Position.X,0,character.HumanoidRootPart.Position.Z)).magnitude
            print(distanceTraveled)
            if distanceTraveled > MAX_SPEED then
                player.Character.HumanoidRootPart.Position = previousPositions[player.Name]
            end
        end

        previousPositions[player.Name] = player.Character.HumanoidRootPart.Position
    end
end

while wait(1) do
    for _, player in pairs(game.Players:GetPlayers()) do
        checkMovement(player)
    end
end 


--remove player from previousPositions table when they leave the game
game.PlayerRemoving:Connect(function(player)
    previousPositions[player.Name] = nil
end)

Previous Script:

game.Players.PlayerAdded:Connect(function(plr)
 if game:GetMarketService("MarketplaceService"):UserOwnsPassAsync(plr.Id, 1743777) then -- what is getmarketservice? Use GetService. Also, its UserOwnsGamePassAsync and plr .Id should be plr.UserId
  plr.CharacterAdded:Connect(function()
   wait()
  char:FindFirstChild("Humanoid").WalkSpeed = 20 
  end
 end
end)

Updated script:

local id = id
game.Players.PlayerAdded:Connect(function(plr)
 if game:GetService("MarketplaceService"):UserOwnsGamePassAsync(plr.UserId, id) then 
  plr.CharacterAdded:Connect(function()
   wait()
   plr.Character:FindFirstChild("Humanoid").WalkSpeed = 20 -- Change this to what walkspeed you want, default is 16.
  end)
 end
end)