Short answer
The seam where collar meets lapel. Height and width of the gorge change the whole character of a jacket.
Where is the gorge seam?
Look at the upper edge of the lapel, where it joins the collar beside the neck. That joining seam is the gorge. On a notched lapel, the notch marks the visible break between collar and lapel; the notch and the joining seam are related details, not interchangeable measurements.
What should I compare at a fitting?
Compare the seam position with the lapel width, collar and jacket button position while wearing the intended shirt. A higher or lower position changes the visible neckline. Bring a reference jacket and point to the part you want changed rather than asking for a particular height without seeing the full proportion.
Compare notch, peak and shawl lapels · Prepare a fitting reference
In bespoke, gorge is not a style preference but a decision made with your tailor — recorded in your Fit Passport so the next commission starts from the same understanding. Ask about it at your fitting; the honest answer includes whether it serves your line or is merely tradition.