I need a spell checker
 

I need a light weight program to do spell checking. Right now I'm running gedit in Gnome in a Debian machine. Can anyone make a wise recommendation?

apt-get install ispell

I did that, but I'm still not able to do a spell check in gedit. The problem is that there is no default language (or no dictionary)installed.

Try using aspell then; apt-get install aspell. Check that libaspell15 is also installed with it as this is the needed plugin for gedit.