Hi Daniel,
If you started your ferment in the glass carboys, with less-than the full gallon of water, then you can safely top up with water now. In the book "First Steps in Winemaking" by your fellow Englishman, CJJ Berry -
he says under 'Conducting your fermentation' - "It is good practice to use all the ingredients for one gallon of must, but make up initially only six to seven pints, which can then be topped up later with water to the full gallon."
You have to leave quite a bit of headroom starting in the glass jar or
you won't have enough air for the yeast to do their aerobic thing -
after they've slowed down to a little hissing simmer, you don't need or
want so much oxygen exposure and then it's important to have the
jar/carboy well topped up.
If you started with the full gallon volume and have so much space due to racking losses, then what Wade says is the better approach than topping up with water - you would be diluting the strength, flavor and quality of the finished wine you have there.
The method I use even with juice as the base instead of fruit is to start the must in a bucket, with say 5-6 quarts so just over a gallon, then after the vigorous ferment is finished in a few days or a week, rack it to the gallon jar, off the initial sediment, and if any is left over (usually there is some excess liquid still), I put that into a smaller bottle - either a regular 750ml wine bottle or even a 1 ltr wine bottle to use for topping up after the next racking(s).
And I'll put my vote in for you getting a hydrometer if you don't have one, it's the best tool for telling you what the wine is doing, or has done - and when it's done.
And, welcome again!
