As to the drains, I think GB and IG are similar. The outside drain does not have a tube but it could. The other way is to drill the hole larger than needed, fill with epoxy, and drill the drainhole in the epoxy, and paint.
You are right to keep the interior to exterior drain tube clear. I use wire. Someone used a drill on one of mine, it penetrated the tube, the water drained into the wood....with predictable consequences.
2005 Integrity 386 "Sojourn"