Okay. So first I'll explain the event.
function limitVehiclesToGang(player)
local userGang = getElementData(player, "gang") -- This needs to be set
local vehicleOwner = getElementData(source, "gang") -- This needs to be set
if vehicleOwner and (userGang ~= vehicleOwner) then
outputChatBox("You don't own this vehicle.", player)
cancelEvent()
end
end
addEventHandler("onClientVehicleEnter", root, limitVehiclesToGang)
You want to modify the message, and make sure that the element data is set.
The GUI seems really simple.