Although the documentation for the drouting module indicates we can assign weights to gateways or carriers, it does not say how.
drouting.xml should be modified to add the specification, and maybe an example:
<listitem><para>list of GWs/carriers (string)
For each ID (GW or carrier) you may specify a weight by appending <quote>=</quote>, followed by a numerical weight, to the <quote>gwid</quote> or the <quote>carrierid</quote>.
Example of list: