Bugs about very long lines not well supported by GtkTextBuffer/GtkTextView are very common.

When loading a very long line, gedit could warn the user that some features may block the user interface.

In this warning info bar, we can imagine a button to split the very long line into lines of 80 characters for example (bonus point: split preferably at word boundaries). This splitting could even be done before the file is loaded into the buffer, so a very long line would never be present in a GtkTextBuffer in the first place. But it requires to stop a file loading in the middle of the operation, send the warning, wait for the response, and then resume the file loading.