I’m used to gThumb to rename lots of pictures from camera’s filename to something including the date, hour taken and a sequence number, like this one: 2011-02-27–16.24.51_007.JPG.
Basically, simply select pictures you want to rename and press F2 (or Edit->Rename).
Previous to version 2.11, gThumb was using a vastly simpler syntax expression for renaming (they name it Template in the Rename dialog). With 2.11 and up, to get the same, I now have to use the following expression:
Please also note that when changing the Rename dialog, they also made it far slower for fair number of pictures (it might take minutes before the Rename dialog appears). For those who know me, I reported the bug in Launchpad as Pictures renaming slower.