Depends on the runway. If a performance limiting runway then gear up as soon as positive climb. If not limiting then gear up after reaching Vyse except that if the runway has plenty of length and it's a no-brainer to land back on, then I'll delay gear up until I:
* can no longer easily land ahead, or
* start a turn, or
* have reached a height that is reasonable for asymmetric ops
which ever of the three happens first. Taking off from some of the looooong runways I've seen (common in the US) I think it's a bad idea not to leave open the option of just landing straight ahead if a donk quits.
Selecting gear up is the demarcation between going to land if the engine fails, or attempting to continue. I emphasize 'attempting' because many light twins aren't required to have any +ve climb performance on one engine, no matter what the configuration.