The contents of the line-tracking buffer (ignoring its gap for a moment, which you could move anywhere) only change when the text-buffer's gap moves. When that happens, each time a line break moves across the gap, you need to update its index in the line tracking buffer.

It doesn't directly matter where the gap is in the line-tracking buffer except ideally it's positioned consistent with the user's cursor so that if they insert a bunch of new lines, they're inserted into the line-tracking gap.

The position of the line-tracking gap doesn't effect the values stored on either side of it gap, so you can still use memmove there.