Try Retext which supports markdown. Retext has been in Ubuntu since 12.04 precise. For earlier Ubuntu releases, or if you want a more recent version of Retext, get it from the project home page or from a PPA.

sudo add-apt-repository ppa:mitya57/ppa

sudo apt-get update

sudo apt-get install retext

This will install Retext.

If you want a gedit plugin with markdown support:

1. Download this and extract the file in ~/Download directory.

   
   



2. Open a terminal, cd to the ~/Downloads/gedit-markdown-master directory and run ./gedit-markdown.sh install.

   
   



3. Restart Gedit and you have a markdown plugin, enable it and enjoy ! .

   
   

Note: You can anytime preview markdown by entering Ctrl+Alt+M.