India taxes the gain, and property was never treaty-exempt
The Singapore side is simple: Singapore has no capital gains tax, so selling your Indian property is not taxed in Singapore at all. The tax is entirely Indian. A long-term gain, on a property held more than two years, is taxed at 12.5 per cent without indexation, and as an NRI you do not get the alternative 20 per cent with indexation that resident sellers have, so your gain is the sale price minus your cost at 12.5 per cent.
There is a specific confusion worth clearing up for Singapore residents. The India-Singapore treaty was famous, for years, for exempting capital gains, and many people still assume it will shelter their Indian sale. But that exemption was about shares, not property, and even for shares it ended: gains on Indian shares bought from April 2017 became taxable in India, with older shares grandfathered. Property is different and always was, under the treaty, gains on immovable property situated in India are taxable in India, so there was never a property exemption to lose. The upshot is that the treaty gives you no relief on a property sale, and the lever is elsewhere, in the withholding.
The withholding trap, and the fix
The trap on any NRI property sale is the withholding. When a non-resident sells, the buyer deducts TDS under Section 195, not the small 1 per cent that applies when the seller is resident, and buyers routinely deduct that TDS on the entire sale price rather than on your actual gain. On a property that has not appreciated much, that means a large part of the whole price is withheld against a much smaller real tax, and you wait to reclaim the difference.
The fix is to get ahead of it with a lower-deduction certificate, applied for before the sale, under what is currently Section 197 and becomes Section 395 under the new law. That tells the buyer to withhold only on your actual gain at the right rate, freeing up the cash the crude withholding would otherwise trap. Alongside it, reinvestment reliefs under Sections 54, 54EC and 54F can reduce or defer the gain if you put the proceeds into another house or specified bonds, and the net proceeds repatriate out of your NRO account within the yearly limit. Getting the certificate, the reliefs and the repatriation right is the whole job on a Singapore-resident sale, and it is the Indian side we handle.