Gtimelog is very simple. It provides no method of editing the log of how your time was spent.
Fortunately the program’s simplicity extends to its system files, which encourage easy manual editing, without the need for a separate interface for doing so. The file format of log files benefits from some explanation however.
In Gtimelog click File -> Edit timelog.txt to view the log for the current day. Each task has only one time associated with it, and that is the time that the task ended. The time that any given task started is taken from the end of the preceeding task.
2012-06-15 09:00: Start
2012-06-15 12:00: Eating mailing list spam
2012-06-15 13:00: **Lunch
In the above timelog.txt, the eating of spam commenced at 09.00, and ended at 12.00.
You can also just open your log file (default is ~/.gtimelog/timelog.txt) with a text editor of your choice, and make the changes there.
Back in gtimelog, you can press Ctrl-R to refresh the display so that it reflects the update. But even if you don’t do that, reports etc. are always generated on the basis of the file, rather than the display, so they’ll include your updates in any case.