Addons is a 'virtual' Gmod folder.
Any "addon/<pack>" would not be used in your path of your web host.
So, if you had on your actual GMod server
/addons/somepack/models/custom1.xxx
/addons/somepack/models/custom2.xxx
on your actual download server, besides /maps/yourcustommap.bz2 (USE COMPRESSION!!! Research using bz2 compression) that you already have working, you would need;
/models/custom1.xxx
/models/custom2.xxx
http://www.google.com/search?lr=&ie=UTF-8&oe=UTF-8&q=how+to+use+sv_downloadurlEven with compressing every file on your fast download server, I don't recommend hosting large mod packs.
There is no way to make it one individual file for sv_downloadurl that would decompress into all models once it got to the clients machine through Gmod/Source.
(If someone can correct me on that, please feel free.)