According to the 1965 edition of The Motor Racing Register:
The GT Manufacturers' Championship
FIA series which replaced the former SportsCar World Championship.
Results decided on a points system as follows:
Le Mans 2 pts
Sebring, Targa Florio, Nurburgring 1000km, Mille Kilometres de Paris, Tour de France 1.6 pts each
Other circuit events 1.3 pts each
Hill climbs and other events included in the Championship for the first time 1 pt each
Classes are : up to 1300cc (GT 1)
Over 1300cc and up to 2000cc (GT 2)
Over 2000cc (GT3)
Each class is sub-divided at 1000cc, 1600cc and 3000cc
1965 Prototype and GT Manufacturers Championship Calendar
Feb 28 Daytona Continental (GT2-3)
Mar 27 Sebring 12 Hours (P, GT 1-2-3)
Apri 11 Bologna Cups (GT 1)
Apr 25 Monza 1000 Kilometres (P, GT 1-2-3)
May 1 Tourist Trophy (GT 3)
May 9 Targa Florio (P, GT 1-2-3)
May 16 Spa Grand Prix (GT2-3)
May 23 Nurburgring 1000 Kilometres (P, GT 1-2-3)
June 6 Mugello (GT 2)
June 23 Rossfeld hill Climb (GT 3)
JUne 19-20 Le Mans 24 Hours (P, GT 1-2-3)
July 3-4 Reims 12 Hours (GT3)
July 4 Bolzano-Mendola hill climb (GT1)
Aug 8 Freiburg hill climb (GT 2)
Aug 15 Enna Cup (GT 3)
Aug 29 Ollon-Villars hill climb (GT 1-2-3)
Sep 5 Nurburgring 500 kilometres (GT 1)
Sep 14-24 Tour de France (GT 1-2-3)
Sep 18-19 Bridgehampton Double 500 (P, GT 1-2-3)
Oct 10 Paris 1000 Kilometres (P, GT 1-2-3)
__________________________________________________________
Perhaps the system shown above is for multiplication of points scored in races, e.g. a win at Le Mans is worth double points and so on.