If the option "Insert spaces instead of tabs" is set on gedit, the latex plugin will still insert a tab when using some of the pugin buttons.
How to replicate:
1 - turn on the option "Insert spaces instead of tabs" on Edit > Preferences > Editor
2 - press tab on a latex document and notice that saces are indeed being used
3 - press the button "Itemize" on the toolbar
4 - notice that the \item line is indented but with a tab rather than spaces
Note: To make easier to see spaces and tabs, active the plugin "Draw spaces"
I'm using gedit latex plugin version 0.2rc3 with gedit 2.30.4 on the latest Debian stable version (Debian Squeeze).