Warp Gates, in general, are either immune or highly resistant to any long-range ships. This includes frigates, etc, to a degree, and also includes snipers and such. You have to get up-close and dirty to actually kill a warp gate. Otherwise, it's far too easy to just snip them off as an afterthought without actually having to do a gate raid. Warp Gates are pretty unique among buildings in this, but they are also pretty unique in their function. And, largely, they are consistent in how they are resistant to long-range shots (or they should be -- if they are not, then it's the other ships that are wrong, not the bombards). Hope that makes sense!