Is there any GST or additional tax that will be charged to the person buying/selling bitcoins on a local exchange?
I'm not 100% sure, but from the guidance it seems that if you are a GST registered person (i.e. company?), then the BTC you sell in Singapore, be it through and exchange or in person, is GST taxable on the full amount (BTC value + commission).
Proper local exchanges will only be charged GST on their commission.