The highlighting looks like this:

Here is how to get this to work:

Install Erlang Language File

1. Download the file erlang.lang.
2. Copy the file to /usr/share/gtksourceview-1.0/language-specs/erlang.lang.
3. Start gedit, open an Erlang file, and choose View > Highlight Mode > Sources > Erlang.

Automatically Recognize *.erl

If you want gedit to automatically recognize that all .erl files should be correctly highlighted, you have to define the mime type (more info is here):

1. Create directory to override mime types, in the command line type

   mkdirhier ~/.local/share/mime/packages

2. Download the custom mime file Override.xml into this directory (if the file already exists, you have to copy the relevant lines by hand):

   cd ~/.local/share/mime/packages
   wget http://martin.ankerl.com/files/Override.xml

3. Update the mime database by merging the file:

   update-mime-database ~/.local/share/mime

4. Restart nautilus (or logout & login again):

   killall nautilus

There you go, Erlang code in all its glory. Happy hacking!