There aren't a lot of structures that have an amortised cost-per-edit; you're really only ever considering the cost of doing a single insert/deletion operation, and you're really only ever trading that performance against the complexity.

A paged gap buffer is actually more like a tree, so instead of a single gap in the middle of a file, you have a gap in the middle of a block, and one block mapped per modified space. Cost of insert/delete is limited to the cost of a memmove within a block (cheap!), and your extent map never grows beyond 2x the number of changes. These upper bounds are incredibly good for edits, and there are only pathological cases that do better.

But what about search?

Search is faster too! Because virtual memory has your entire file contiguous, a search is as fast as a scan[1], which might embolden you to try indexing your file, which might really impress your users.

[1]: https://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm