On the short term two gallons of head space doesn’t make much difference. The active release of CO2 acts as an inert gas.
My Option one, is to fill the six gallon carboy with juice from the freezer. This might result in a mix like Briana grape/ Red grape, ,, or apple/ red grape, ,, or mulberry red grape. This would require adding tannins to match the red grape tannic flavor. ,,, With Petite Pearl last year I intended a rose, 80% whites and 17% Petite Pearl steeped on the skins. What resulted is that the tannins and color of the PP dominated everything producing a red wine. (By the way the PP will make a better tannin in Marquette/ ie a good blend) If I had downsized to a condo-old-fart I might take a grape concentrate from the grocery store.
Option two is put it in a five gallon and top it off with a similar wine. (My normal racking from a full six gallon would be into a five, ,, at some point you will need a five gallon).
Option three which most of us don’t do is fill the void with a water filled bag. Some of us do marbles and I have a few LDPE carboy filler rods which are mostly used on the third racking when volume is similar.
Option four when you have the pressed pulp is to do a very quick second wine for topping off/ possibly with grocery store white grape juice.
Option five I have seen is to fill the void with a bland juice like zucchin. We have a member in Wisconsin Vinters who uses zucchini on all his topping off issues