Why is Gemini allowed for buy/sell of crypto, but Coinbase not allowed in SG?

It's probably just that they haven't applied to operate in Singapore yet, rather than any incompetency in their exchange operations or failure in security etc. Considering Singapore is so small anyways, I'm sure we are not that high on the list of priorities hahah. Anyways if you want a good and cheap exchange to use, you can always use Binance and use DBS Remit to get your money into the exchange or through P2P trading on Binance.