GDV is gross development value, which is just what the place will be worth once you've finished the work. It's the first figure in any appraisal and pretty much everything else hangs off it. Take the GDV, take away what you pay for the property, the build, stamp duty, finance, fees and the cost of selling, and what's left is your profit. So if the GDV is out, your profit is out by the same amount.
To get to a sensible number you need to look at what similar properties have sold for recently. It has to be sold prices rather than asking prices. Land Registry has every completed sale in England and Wales and you can look them up for free, though it takes a while if you're going street by street.
Finding good comparables is harder than it sounds. You want the same type of property in the same area, in roughly the condition yours will be in once it's done, sold in the last year or so. In practice you rarely get all of that. You end up with a handful of sales that are each a bit different to yours and you make a judgement. That's fine, as long as you're honest about which way the differences cut.
Size is the one people underestimate. An area average for detached houses can be pulled up or down by a few very big or very small ones, and if yours isn't typical the average won't tell you much.
What you're doing to the building matters too, though less predictably. Converted flats often sell for a little under similar purpose built ones. New builds can get a premium, partly because of the warranty, but that isn't guaranteed everywhere and some buyers actively avoid them. A good refurb should put you somewhere around the local median, give or take.
If there's one thing to be careful of it's being optimistic. Knock 10% off your GDV and see what happens to your margin. On a lot of deals it roughly halves, and missing a sale price by 10% isn't unusual.
For what it's worth, Avalor does the comparables part for you. Put in a postcode and it pulls recent Land Registry sales for that property type, gives you the median and tells you how many sales it's based on, so you know how much to trust it. If you think your place is worth more or less you can put your own figure in.