Not sure about the restrictions, but they do have value for every company doing business with Singapore (and that's quite some good nr of companies). In Singapore .com.sg is as widely used as .COM or any generic TLD. I can know, as I work for a company with head offices in Singapore and we're in touch with the Singapore folks daily and have a nice chat with them often... Singapore domains definitely have their value.
Stay away from .gs -- no one in Singapore will just find it logical to switch the characters in their extention. Also, .gs is the South Georgia/South Sandwich Islands, population is approx 20 people that live there permanently, which is less than the average floor of a Singapore appartments building.
Stick with the .com.sg if you want to aim at the Singapore market.