To make a use of @math in dust templates you need to add dust helpers which is by default excluded from dust core package.
The specific syntax that you need in order to "load" the dust helpers in node is:
var dust = require('dustjs-linkedin');
dust.helper = require('dustjs-helpers');
Incase you cannot add these helpers, which would be really strange decision, still you can create your own function in current code base like this which can be used instead of @gt or @math
var baseContext = dust.makeBase({
position: function(chunk, context) {
return context.stack.index + 1;
},
});
Now you can use {position} instead of ${idx} which will count the loop from 1 to n.