Your build profile needs a packages section which will tell the build where the gridx code is located.
build.profile.js
basePath: "../dojo-release-1.8.3-src",
...
packages:[
{name: 'dojo', location: 'dojo'},
{name: 'dijit', location: 'dijit'},
{name: 'dojox', location: 'dojox'},
{name: 'gridx', location: '../PATH/TO/gridx'} // path is relative from basePath
],