There's currently no way to control the Gutter width. If you'd like to see this, please add a Feature Request on the project's SourceForge page
so it can be tracked.
That said, if you're cool with hacking the source yourself, the only part of the Gutter that's variable width is the line number list. I can see where it could be considered jarring, especially when it resizes when going from 9 lines to 10. You can modify org.fife.ui.rtextarea.LineNumberList.java, changing the updateCellWidths() method to always return a fixed size.