Instead of using "Syntax" menu you can also manually source in the syntax file. Edit the file with gvim and at : (colon) command give 'so' command. For example -
gvim foo.pc :so $VIM/syntax/esqlc.vim
Each syntax file supports a default file extension names, for example, Javascript syntax file supports *.js extension. If you happen to use different extension conflicting with another default syntax file as in putting Javascript in *.html file than you MUST source in with command :so $VIM/syntax/javascript.vim. Best method is create a soft link like -
ln -s $VIM/syntax/javascript.vim js gvim foo.html (... this file contains javascript functions and HTML) :so js