Git author and committer can be different, and so can author and commit dates (i.e. someone might have re-committed earlier changes to a new repository, or imported them from another VCS). So Gource should show author date (it already shows the author name and not the committer name).
Use author date, not commit date, for Git log.
Hi. The reason for using the commit date was that author dates, as you mentioned, may not be in the same order as commits in the Git log, causing a lot of 'time travel' in the displayed time line.
If the log was sorted into author date order as well, either by a different log command or by a pre sorting phase added to Gource, then I would be ok with perhaps using the author date.
This can be done using the "--author-date-order" option in git-log. (I just had a similar problem with a repository where an SVN migration had broken all the commit dates.)
Unfortunately, there doesn't seem to be an option to filter by author date the way --since/--until. So --start-date and --stop-date would still filter by commit date.