The attached file is a rewrite of the GtkSourceView 3.0 (the syntax highlighting control used by Gedit) asm-intel.lang file that adds support for NASM & NASM-X.
To install on a Linux or BSD system, download the asm-intel.lang file in the attachments and enter the following command into your command line:
sudo cp asm-intel.lang $(locate "3.0/language-spec" | grep -v "specs/")
This will copy the file into the GtkSourceView 3.0 language-spec directory (it's in different places on different distro's).
If you're on windows, then you might look where ever Gedit was installed and search for the language-spec directory.. just make sure the file goes into the gtksourceview-3.0 directory instead of the 2.0 (I think gedit actually supports both file types and this is for 3.0).