ACE is a powerful editor for configuring Gajim. It holds all the preferences and settings of the Preferences window and many additional settings.
To access ACE, first open the Preferences window (for example by hitting Ctrl + P), then click on the "Advanced" Tab.
Click the "Open" button in "Advanced Configuration Editor" section.
There, you can change a lot of options concerning Gajim. You have to double-click the "Value" column of the line to modify it. If it is a boolean (True or False), it will toggle it to the opposite value. If it's not boolean, you will have to modify the value by hand (e.g. type in the desired number, side, ...).
Changing values via ACE can render Gajim unusable, so be careful! If Gajim happens to be unusable after you changed some values, you can always remove or edit your config in ~/.gajim/config. But be careful! Removing your config file means you have to start from scratch (add accounts, etc.). You will not lose your history logs though.