I don't think there should be a problem with line numbers. I would make two helper arrays containing the indices of the new-line characters, corresponding to the two gap buffer text arrays (new-line positions are sorted ascending for the first array, descending for the second array).

Speaking as someone who's gone all the way from implementing a Red-black tree to making a rope data structure using the RB tree, to making a text editor that can edit almost arbitrarily large text files (dozens of gigabytes) without user-perceivable latency ;-)