a gate which switches to 2 after anything passed 1

Sorry for the minimal (and useless) answer. there are many ways to perform an action only one time. Most obvious object for that is [onebang], which needs a bang before it will allow exactly one bang to pass. Or a gate can close itself or switch output if something passed the output. Always [trigger] is involved when the order of actions is important, and that is very often the case.