The conversion charge is actually the cost of minting the gold wafers from Switzerland as well as for transportation and insurance.
The price of gold quoted daily will be pegged to the international gold price and will definitely be better than goldsmith shops without the usual "workmanship" charges.
When you purchases gold through GSPA, this is recorded into a passbook. The Bank actually keeps the gold in safe custody for you, and as such, interest is not paid.
The Bank will usually charge an administrative fee for this service at 0.25% p.a. on the highest gold balance per month. Maybank, however, does not levy this 0.25% administrative charge for the GSPA.