Se o gedit tiver plug-ins ausentes, verifique se você tem o gedit-plugins
instalado.
sudo apt-get install gedit-plugins
Depois disso, (você provavelmente precisará reiniciar o gedit) você terá um plugin chamado "Text Size". Verifique se o plug-in está ativado e você poderá "aplicar zoom" (ou seja, alterar o tamanho da fonte) usando Ctrl + roda do mouse.
Esta é a lista de plugins disponíveis no pacote gedit-plugins
$ apt-cache show gedit-plugins
[...]
The following plugins are included:
* Git: Git differences
* Bookmarks: Easy document navigation with bookmarks
* Bracket Completion: Automatically adds closing brackets.
* Character Map: Insert special characters just by clicking on them.
* Code comment: Comment out or uncomment a selected block of code.
* Color Picker: Pick a color from a dialog and insert its hexadecimal representation.
* Color Scheme Editor: Source code color scheme editor
* Commander: Command line interface for advanced editing
* Dashboard: A Dashboard for new tabs
* Draw Spaces: Draw Spaces and Tabs
* Git: Git differences
* Join/Split Lines: Join several lines or split long ones
* Multi Edit: Edit document in multiple places at once
* Smart Spaces: Forget you're not using tabulations.
* SyncTeX: Synchronize between LaTeX and PDF with gedit and evince.
* Embedded Terminal: Embed a terminal in the bottom pane.
* Text Size: Easily increase and decrease the text size
* Word Completion: Word completion using the completion framework
[...]