Create a block with aaatrigger texture. If the block is an entity the texture will be automaticaly invisible.
Then turn the block to entity, select the trigger_gravity and go to Gravity option. Then here you can choose the gravity ammount. The number you will write will multiply the gravity ammount of the server (sv_gravity, default is 800). So if you want the zone to be half of the normal gravity write 0.5, 0.5 x 800 = 400, you will have 400 gravity into this cube.