How do you get gloss paint off an enamel sign without ruining the enamel?

No sure way, be prepared to reapply the enamel, but you can try a damp (not dripping wet) rag of solvent such as lacquer thinner or mineral spirits. Try the mineral spirits first due to the volatility of the lacquer thinner. Last resort use Methyl Ethyl Keytone or Acetone...both of these will strip the paint off but you have to work fast and be very careful. All the above applies only if the paint you are trying to remove is quick-drying lacquer based, otherwise you just have to sand and repaint.How do you get gloss paint off an enamel sign without ruining the enamel?
I think David is all correct for enamel or oil base......If the gloss is water base just try warm soap and water first. You need to soften up the layer of paint you want to remove.