Clean and wash your grapes well. Deskin the grapes. Combine the pulp, crushed ginger and 2 1/2 cups of sugar in a thick bottomed vessel and keep aside.
Add the 1/2 cup sugar to the skins of the grapes and blend it in the mixer to form a smooth paste. Add a little wine if you think its needed.
Add the above to the grape pulp and bring to a boil. Once the sugar melts completely, reduce the flame and let it simmer for 8 to 10 minutes till the fruits start to disintegrate and looks pulpy. Turn off the flame and let the mixture cool down a little.
Sieve the mixture to get rid of all the seeds and thick skin. Since we have already ground the skins there will not be too much to be sieved. Press as much of the pulp of the fruits as possible with the back of your spoon to get all the juices of of the fruit. I let the pulp hang in muslin cloth for an hour to let all the juice come out.
Place the thick pulp back on the stove after adding the wine to it and let it simmer slowly, till it reaches the jammy consistency. Mine took about 45 mins or so. A good test to check whether the jam is done to pour a drop of the hot jam on a chilled plate, let it rest for 30 seconds and then tilt the plate, the jam should flow slowly and feel thick.
Store it in an sterilized airtight bottle.